Messages in this thread |  | | | Date | Wed, 28 Apr 2010 12:30:58 +1000 | | From | Dave Chinner <> | | Subject | Re: [PATCH] XFS: Let the broken fiemap work in query mode. |
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On Wed, Apr 28, 2010 at 10:00:01AM +0800, Tao Ma wrote: > 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.
Applications need to handle mappings changing from query to getting the mapping, so this should not be a major issue. Especially as the method of fiemap indicating there are more extents to be extracted from the inode in the case the kernel can't allocate a buffer large enough is already documented.
Realistically though, xfs_getbmap() needs a complete rewrite so right now I'd prefer just to do the minimum to fix the reported problem that continue to make it into even more of a mess than it is now...
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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