On Fri, Oct 18, 2019 at 10:16 AM Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
>
> On Fri, Oct 18, 2019 at 09:10:34AM -0700, Dan Williams wrote:
> > Hi,
> >
> > In the course of tracking down a v5.3 regression with filesystem-dax
> > unable to generate huge page faults on any filesystem, I found that I
> > can't generate huge faults on v5.2 with xfs, but ext4 works. That
> > result indicates that the block device is properly physically aligned,
> > but the allocator is generating misaligned extents.
> >
> > The test fallocates a 1GB file and then looks for a 2MB aligned
> > extent. However, fiemap reports:
> >
> > for (i = 0; i < map->fm_mapped_extents; i++) {
> > ext = &map->fm_extents[i];
> > fprintf(stderr, "[%ld]: l: %llx p: %llx len: %llx flags: %x\n",
> > i, ext->fe_logical, ext->fe_physical,
> > ext->fe_length, ext->fe_flags);
> > }
> >
> > [0]: l: 0 p: 208000 len: 1fdf8000 flags: 800
> > [1]: l: 1fdf8000 p: c000 len: 170000 flags: 800
> > [2]: l: 1ff68000 p: 2000c000 len: 1ff70000 flags: 800
> > [3]: l: 3fed8000 p: 4000c000 len: 128000 flags: 801
> >
> > ...where l == ->fe_logical and p == ->fe_physical.
> >
> > I'm still searching for the kernel where this behavior changed, but in
> > the meantime wanted to report this in case its something
> > straightforward in the allocator. The mkfs.xfs invocation in this case
> > was:
> >
> > mkfs.xfs -f -d su=2m,sw=1 -m reflink=0 /dev/pmem0
>
> As we talked about on irc while I waited for a slooow imap server, I
> think this is caused by fallocate asking for a larger allocation than
> the AG size. The allocator of course declines this, and bmap code is
> too fast to drop the alignment hints. IIRC Brian and Carlos and Dave
> were working on something in this area[1] but I don't think there's been
> any progress in a month(?)
>
> Then Dan said agsize=131072, which means 512M AGs, so a 1G fallocate
> will never generate an aligned allocation... but a 256M one seems to
> work fine on my test vm.
>
Thanks Darrick. While reducing the fallocate causes physical alignment
to happen some extents are still misaligned to the logical offset, but
adding agcount=2 cleans it up for me.
