Messages in this thread | | | Date | Wed, 28 Apr 2010 10:00:01 +0800 | From | Tao Ma <> | Subject | Re: [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.
Regards, Tao
| |