8:17 a.m.
On Thu, Mar 24, 2016 at 01:51:12PM +0100, Jan Kara wrote:
On Wed 23-03-16 11:39:45, Ross Zwisler wrote:
> On Mon, Mar 21, 2016 at 02:22:49PM +0100, Jan Kara wrote:
> > When a fault to a hole races with write filling the hole, it can happen
> > that block zeroing in __dax_fault() overwrites the data copied by write.
> > Since filesystem is supposed to provide pre-zeroed blocks for fault
> > anyway, just remove the racy zeroing from dax code. The only catch is
> > with read-faults over unwritten block where __dax_fault() filled in the
> > block into page tables anyway. For that case we have to fall back to
> > using hole page now.
> >
> > Signed-off-by: Jan Kara <jack(a)suse.cz>
> > ---
> > fs/dax.c | 9 +--------
> > 1 file changed, 1 insertion(+), 8 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index d496466652cd..50d81172438b 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -582,11 +582,6 @@ static int dax_insert_mapping(struct inode *inode, struct
buffer_head *bh,
> > error = PTR_ERR(dax.addr);
> > goto out;
> > }
> > -
> > - if (buffer_unwritten(bh) || buffer_new(bh)) {
> > - clear_pmem(dax.addr, PAGE_SIZE);
> > - wmb_pmem();
> > - }
>
> I agree that we should be dropping these bits of code, but I think they are
> just dead code that could never be executed? I don't see how we could have
> hit a race?
>
> For the above, dax_insert_mapping() is only called if we actually have a block
> mapping (holes go through dax_load_hole()), so for ext4 and XFS I think
> buffer_unwritten() and buffer_new() are always false, so this code could never
> be executed, right?
>
> I suppose that maybe we could get into here via ext2 if BH_New was set? Is
> that the race?
Yeah, you are right that only ext2 is prone to the race I have described
since for the rest this should be just a dead code. I'll update the changelog
in this sense.
What do you think about updating ext2 so that like ext4 and xfs it doesn't
ever return BH_New? AFAICT ext2 doesn't rely on DAX to clear the sectors it
returns - it does that in ext2_get_blocks() via dax_clear_sectors(), right?
Or, really, I guess we could just leave ext2 alone and let it return BH_New,
and just make sure that DAX doesn't do anything with it.
> > if (vmf->flags & FAULT_FLAG_WRITE) {
> > error = get_block(inode, block, &bh, 1);
> > count_vm_event(PGMAJFAULT);
> > @@ -950,8 +945,6 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned
long address,
> > }
> >
> > if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> > - clear_pmem(dax.addr, PMD_SIZE);
> > - wmb_pmem();
> > count_vm_event(PGMAJFAULT);
> > mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
> > result |= VM_FAULT_MAJOR;
>
> I think this whole block is just dead code, right? Can we ever get into here?
>
> Same argument applies as from dax_insert_mapping() - if we get this far then
> we have a mapped buffer, and in the PMD case we know we're on ext4 of XFS
> since ext2 doesn't do huge page mappings.
>
> So, buffer_unwritten() and buffer_new() both always return false, right?
>
> Yea...we really need to clean up our buffer flag handling. :)
Hum, looking at the code now I'm somewhat confused. __dax_pmd_fault does:
if (!write && !buffer_mapped(&bh) && buffer_uptodate(&bh)) {
... install zero page ...
}
but what the buffer_update() check is about? That will never be true,
right? So we will fall back to the second branch and there we can actually
hit the
if (buffer_unwritten(&bh) || buffer_new(&bh)) {
because for read fault we can get unwritten buffer. But I guess that is a
mistake in the first branch. After fixing that we can just remove the
second if as you say. Unless you object, I'll update the patch in this
sense.
I can't remember if I've ever seen this code get executed - I *think* that
when we hit a hole we always drop back and do 4k zero pages via this code:
/*
* If the filesystem isn't willing to tell us the length of a hole,
* just fall back to PTEs. Calling get_block 512 times in a loop
* would be silly.
*/
if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) {
dax_pmd_dbg(&bh, address, "allocated block too small");
return VM_FAULT_FALLBACK;
}
I agree that this could probably use some cleanup and additional testing.