1:15 a.m.
I've been writing a patch to migrate the defined DAX_ZERO_PAGE
to XA_ZERO_ENTRY for representing holes in files. XA_ZERO_ENTRY
is defined in include/linux/xarray.h, where it's defined using
xa_mk_internal(257). This function returns a void pointer, which
is incompatible with the bitwise arithmetic it is performed on with.
Currently, DAX_ZERO_PAGE is defined as an unsigned long,
so I considered typecasting it. Typecasting every time would be
repetitive and inefficient. I thought about making a new definition
for it which has the typecast, but this breaks the original point of
using already defined terms.
Should we go the route of adding a new definition, we might as
well just change the definition of DAX_ZERO_PAGE. This would
break the simplicity of the current DAX bit definitions:
#define DAX_LOCKED (1UL << 0)
#define DAX_PMD (1UL << 1)
#define DAX_ZERO_PAGE (1UL << 2)
#define DAX_EMPTY (1UL << 3)
Any thoughts on this, and what could be the best solution here?
Best regards,
Amy Parker
(they/them)
1:33 a.m.
On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
I've been writing a patch to migrate the defined DAX_ZERO_PAGE
to XA_ZERO_ENTRY for representing holes in files.
Why? IIRC XA_ZERO_ENTRY ("no mapping in the address space") isn't the
same as DAX_ZERO_PAGE ("the zero page is mapped into the address space
because we took a read fault on a sparse file hole").
--D
> XA_ZERO_ENTRY
> is defined in include/linux/xarray.h, where it's defined using
> xa_mk_internal(257). This function returns a void pointer, which
> is incompatible with the bitwise arithmetic it is performed on with.
>
> Currently, DAX_ZERO_PAGE is defined as an unsigned long,
> so I considered typecasting it. Typecasting every time would be
> repetitive and inefficient. I thought about making a new definition
> for it which has the typecast, but this breaks the original point of
> using already defined terms.
>
> Should we go the route of adding a new definition, we might as
> well just change the definition of DAX_ZERO_PAGE. This would
> break the simplicity of the current DAX bit definitions:
>
> #define DAX_LOCKED (1UL << 0)
> #define DAX_PMD (1UL << 1)
> #define DAX_ZERO_PAGE (1UL << 2)
> #define DAX_EMPTY (1UL << 3)
>
> Any thoughts on this, and what could be the best solution here?
>
> Best regards,
> Amy Parker
> (they/them)
1:44 a.m.
On Sun, Nov 8, 2020 at 5:35 PM Darrick J. Wong <darrick.wong(a)oracle.com> wrote:
On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> I've been writing a patch to migrate the defined DAX_ZERO_PAGE
> to XA_ZERO_ENTRY for representing holes in files.
Why? IIRC XA_ZERO_ENTRY ("no mapping in the address space") isn't the
same as DAX_ZERO_PAGE ("the zero page is mapped into the address space
because we took a read fault on a sparse file hole").
--D
Matthew recommended that we use a single entry here instead of adding
extra unnecessary definitions:
Right now, fs/dax.c uses a bit -- DAX_ZERO_PAGE --
to represent a hole in the file. It could equally well use a single entry,
and we already have one defined, XA_ZERO_ENTRY. I think a patch to
convert it over would be a great idea.
In practice, it works perfectly fine - ran a qemu instance with a 20 GiB
NVDIMM, set up a standard namespace and ran XFS with DAX enabled
on it, and then passed over it with xfstests. Nothing changed versus
the current effect.
1:50 a.m.
On Sun, Nov 08, 2020 at 05:33:22PM -0800, Darrick J. Wong wrote:
On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> I've been writing a patch to migrate the defined DAX_ZERO_PAGE
> to XA_ZERO_ENTRY for representing holes in files.
Why? IIRC XA_ZERO_ENTRY ("no mapping in the address space") isn't the
same as DAX_ZERO_PAGE ("the zero page is mapped into the address space
because we took a read fault on a sparse file hole").
There's no current user of XA_ZERO_ENTRY in i_pages, whether it be
DAX or non-DAX.
> XA_ZERO_ENTRY
> is defined in include/linux/xarray.h, where it's defined using
> xa_mk_internal(257). This function returns a void pointer, which
> is incompatible with the bitwise arithmetic it is performed on with.
We don't really perform bitwise arithmetic on it, outside of:
static int dax_is_zero_entry(void *entry)
{
return xa_to_value(entry) & DAX_ZERO_PAGE;
}
> Currently, DAX_ZERO_PAGE is defined as an unsigned long,
> so I considered typecasting it. Typecasting every time would be
> repetitive and inefficient. I thought about making a new definition
> for it which has the typecast, but this breaks the original point of
> using already defined terms.
>
> Should we go the route of adding a new definition, we might as
> well just change the definition of DAX_ZERO_PAGE. This would
> break the simplicity of the current DAX bit definitions:
>
> #define DAX_LOCKED (1UL << 0)
> #define DAX_PMD (1UL << 1)
> #define DAX_ZERO_PAGE (1UL << 2)
> #define DAX_EMPTY (1UL << 3)
I was proposing deleting the entire bit and shifting DAX_EMPTY down.
1:54 a.m.
On Sun, Nov 8, 2020 at 5:50 PM Matthew Wilcox <willy(a)infradead.org> wrote:
On Sun, Nov 08, 2020 at 05:33:22PM -0800, Darrick J. Wong wrote:
> On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> > I've been writing a patch to migrate the defined DAX_ZERO_PAGE
> > to XA_ZERO_ENTRY for representing holes in files.
>
> Why? IIRC XA_ZERO_ENTRY ("no mapping in the address space") isn't
the
> same as DAX_ZERO_PAGE ("the zero page is mapped into the address space
> because we took a read fault on a sparse file hole").
There's no current user of XA_ZERO_ENTRY in i_pages, whether it be
DAX or non-DAX.
> > XA_ZERO_ENTRY
> > is defined in include/linux/xarray.h, where it's defined using
> > xa_mk_internal(257). This function returns a void pointer, which
> > is incompatible with the bitwise arithmetic it is performed on with.
We don't really perform bitwise arithmetic on it, outside of:
static int dax_is_zero_entry(void *entry)
{
return xa_to_value(entry) & DAX_ZERO_PAGE;
}
We also have:
if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
unsigned long index = xas->xa_index;
/* we are replacing a zero page with block mapping */
if (dax_is_pmd_entry(entry))
unmap_mapping_pages(mapping, index & ~PG_PMD_COLOUR,
PG_PMD_NR, false);
else /* pte entry */
unmap_mapping_pages(mapping, index, 1, false);
}
and:
*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
DAX_PMD | DAX_ZERO_PAGE, false);
> > Currently, DAX_ZERO_PAGE is defined as an unsigned long,
> > so I considered typecasting it. Typecasting every time would be
> > repetitive and inefficient. I thought about making a new definition
> > for it which has the typecast, but this breaks the original point of
> > using already defined terms.
> >
> > Should we go the route of adding a new definition, we might as
> > well just change the definition of DAX_ZERO_PAGE. This would
> > break the simplicity of the current DAX bit definitions:
> >
> > #define DAX_LOCKED (1UL << 0)
> > #define DAX_PMD (1UL << 1)
> > #define DAX_ZERO_PAGE (1UL << 2)
> > #define DAX_EMPTY (1UL << 3)
I was proposing deleting the entire bit and shifting DAX_EMPTY down.
That'd probably be a better idea - so what should we do about the type
issue? Not typecasting it causes it not to compile.
3:35 a.m.
On Sun, Nov 08, 2020 at 05:54:14PM -0800, Amy Parker wrote:
On Sun, Nov 8, 2020 at 5:50 PM Matthew Wilcox
<willy(a)infradead.org> wrote:
>
> On Sun, Nov 08, 2020 at 05:33:22PM -0800, Darrick J. Wong wrote:
> > On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> > > XA_ZERO_ENTRY
> > > is defined in include/linux/xarray.h, where it's defined using
> > > xa_mk_internal(257). This function returns a void pointer, which
> > > is incompatible with the bitwise arithmetic it is performed on with.
>
> We don't really perform bitwise arithmetic on it, outside of:
>
> static int dax_is_zero_entry(void *entry)
> {
> return xa_to_value(entry) & DAX_ZERO_PAGE;
> }
We also have:
if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
unsigned long index = xas->xa_index;
/* we are replacing a zero page with block mapping */
if (dax_is_pmd_entry(entry))
unmap_mapping_pages(mapping, index & ~PG_PMD_COLOUR,
PG_PMD_NR, false);
else /* pte entry */
unmap_mapping_pages(mapping, index, 1, false);
}
and:
*entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
DAX_PMD | DAX_ZERO_PAGE, false);
Right. We need to be able to distinguish whether an entry represents
a PMD size. So maybe we need XA_ZERO_PMD_ENTRY ... ? Or we could use
the recently-added xa_get_order().
> > > Should we go the route of adding a new definition, we
might as
> > > well just change the definition of DAX_ZERO_PAGE. This would
> > > break the simplicity of the current DAX bit definitions:
> > >
> > > #define DAX_LOCKED (1UL << 0)
> > > #define DAX_PMD (1UL << 1)
> > > #define DAX_ZERO_PAGE (1UL << 2)
> > > #define DAX_EMPTY (1UL << 3)
>
> I was proposing deleting the entire bit and shifting DAX_EMPTY down.
That'd probably be a better idea - so what should we do about the type
issue? Not typecasting it causes it not to compile.
I don't think you'll need to do any casting once the bit operations go
away ...
4:41 p.m.
Sorry for the long reply time - personal issues came up.
On Sun, Nov 8, 2020 at 7:36 PM Matthew Wilcox <willy(a)infradead.org> wrote:
On Sun, Nov 08, 2020 at 05:54:14PM -0800, Amy Parker wrote:
> On Sun, Nov 8, 2020 at 5:50 PM Matthew Wilcox <willy(a)infradead.org> wrote:
> >
> > On Sun, Nov 08, 2020 at 05:33:22PM -0800, Darrick J. Wong wrote:
> > > On Sun, Nov 08, 2020 at 05:15:55PM -0800, Amy Parker wrote:
> > > > XA_ZERO_ENTRY
> > > > is defined in include/linux/xarray.h, where it's defined using
> > > > xa_mk_internal(257). This function returns a void pointer, which
> > > > is incompatible with the bitwise arithmetic it is performed on with.
> >
> > We don't really perform bitwise arithmetic on it, outside of:
> >
> > static int dax_is_zero_entry(void *entry)
> > {
> > return xa_to_value(entry) & DAX_ZERO_PAGE;
> > }
>
> We also have:
>
> if (dax_is_zero_entry(entry) && !(flags & DAX_ZERO_PAGE)) {
> unsigned long index = xas->xa_index;
> /* we are replacing a zero page with block mapping */
> if (dax_is_pmd_entry(entry))
> unmap_mapping_pages(mapping, index & ~PG_PMD_COLOUR,
> PG_PMD_NR, false);
> else /* pte entry */
> unmap_mapping_pages(mapping, index, 1, false);
> }
>
> and:
>
> *entry = dax_insert_entry(xas, mapping, vmf, *entry, pfn,
> DAX_PMD | DAX_ZERO_PAGE, false);
Right. We need to be able to distinguish whether an entry represents
a PMD size. So maybe we need XA_ZERO_PMD_ENTRY ... ? Or we could use
the recently-added xa_get_order().
I could add an additional dependent patch for this. Where would we
want XA_ZERO_PMD_ENTRY declared? Considering we're dependent
on DAX_PMD, I'd say in fs/dax.c, but if there's a better solution I'm
missing...
>
> That'd probably be a better idea - so what should we do about the type
> issue? Not typecasting it causes it not to compile.
I don't think you'll need to do any casting once the bit operations go
away ...
True, but what're we going to do about dax_is_zero_entry? We haven't
figured out what to do about that yet... a typecast back to void* of
xa_to_value locally could work, as it itself is just shifting an entry right
by 1 bit and then typecasting it to unsigned long. Thoughts?
Best regards,
Amy Parker
(she/her)
584
days inactive
602
days old
6 comments
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participants (3)
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Amy Parker -
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Matthew Wilcox