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    SubjectRe: [PATCH] XFS: Let the broken fiemap work in query mode.
    Hi Dave,

    Dave Chinner wrote:
    > On Tue, Apr 27, 2010 at 02:17:45PM +0800, Tao Ma wrote:
    >> According to Documentation/filesystems/fiemap.txt, If fm_extent_count
    >> is zero, then the fm_extents[] array is ignored (no extents will be
    >> returned), and the fm_mapped_extents count will hold the number of
    >> extents needed.
    >> But as the commit 97db39a1f6f69e906e98118392400de5217aa33a has changed
    >> bmv_count to the caller's input buffer, this number query function can't
    >> work any more. As this commit is written to change bmv_count from
    >> MAXEXTNUM because of ENOMEM, we can't find a really suitable number to
    >> set bmv_count now in xfs_vn_fiemap. Since we really have no idea of how
    >> much extents the file has, a big number may cause ENOMEM, while a small
    >> one will mask the real extent no.
    >> So this patch try to resolve this problem by adding a temporary getbmapx
    >> in xfs_getbmap. If the caller didn't give bmv_count, we don't allocate
    >> the "out" either. Instead, every time we want to use 'out', use '&tmp'
    >> instead.
    >> I know this solution is a bit ugly, but I can't find a way to resolve
    >> this issue while not changing the codes too much. So any good suggestion
    >> is welcomed.
    > I don't see a need to change xfs_getbmap() to fix this. We can limit
    > the maximum allocation size to something realistic just by setting
    > bm.bmv.count to something sane. e.g, in xfs_vn_fiemap:
    > - bm.bmv_count = fieinfo->fi_extents_max + 1;
    > + bm.bmv.count = !fieinfo->fi_extents_max ? MAXEXTNUM :
    > + fieinfo->fi_extents_max - 1;
    > + bm.bmv_count = MIN(bm.bmv_count,
    > (PAGE_SIZE * 16 / sizeof(struct getbmapx)));
    > Unless I'm missing something, that should also prevent the case of
    > an application providing a really large fi_extents_max from
    > triggering ENOMEM in most cases as well.
    I just worry about one thing: What if the real extent number is larger
    than the PAGE_SIZE * 16 / sizeof(struct getbmapx)? In this case, we will
    give up the wrong extent number to the user space.
    > FWIW, how did you find this? Is it possible for you to add a test
    > for this regression into xfstests so that we don't break it again
    > in future?
    Sure, I will check and see whether I can add it in xfstests.


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