| Message ID | 1484404393-6413-1-git-send-email-hch@lst.de (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
On Sat, Jan 14, 2017 at 03:33:13PM +0100, Christoph Hellwig wrote: > Due to the way how xfs_iomap_write_allocate tries to convert the whole > found extents from delalloc to real space we can run into a race > condition with multiple threads doing writes to this same extent. > For the non-COW case that is harmless as the only thing that can happen > is that we call xfs_bmapi_write on an extent that has already been > converted to a real allocation. For COW writes where we move the extent > from the COW to the data fork after I/O completion the race is, however, > not quite as harmless. In the worst case we are now calling > xfs_bmapi_write on a region that contains hole in the COW work, which > will trip up an assert in debug builds or lead to file system corruption > in non-debug builds. This seems to be reproducible with workloads of > small O_DSYNC write, although so far I've not managed to come up with > a with an isolated reproducer. > > The fix for the issue is relatively simple: tell xfs_bmapi_write > that we are only asked to convert delayed allocations and skip holes > in that case. > > Signed-off-by: Christoph Hellwig <hch@lst.de> > --- > fs/xfs/libxfs/xfs_bmap.c | 41 +++++++++++++++++++++++++++++------------ > fs/xfs/libxfs/xfs_bmap.h | 3 +++ > fs/xfs/xfs_iomap.c | 2 +- > 3 files changed, 33 insertions(+), 13 deletions(-) > > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c > index 44773c9..ceae779 100644 > --- a/fs/xfs/libxfs/xfs_bmap.c > +++ b/fs/xfs/libxfs/xfs_bmap.c > @@ -4514,8 +4514,6 @@ xfs_bmapi_write( > int n; /* current extent index */ > xfs_fileoff_t obno; /* old block number (offset) */ > int whichfork; /* data or attr fork */ > - char inhole; /* current location is hole in file */ > - char wasdelay; /* old extent was delayed */ > > #ifdef DEBUG > xfs_fileoff_t orig_bno; /* original block number value */ > @@ -4603,22 +4601,41 @@ xfs_bmapi_write( > bma.firstblock = firstblock; > > while (bno < end && n < *nmap) { > - inhole = eof || bma.got.br_startoff > bno; > - wasdelay = !inhole && isnullstartblock(bma.got.br_startblock); > + bool need_alloc = false, wasdelay = false; > > - /* > - * Make sure we only reflink into a hole. > - */ > - if (flags & XFS_BMAPI_REMAP) > - ASSERT(inhole); > - if (flags & XFS_BMAPI_COWFORK) > - ASSERT(!inhole); > + /* in hole or beyoned EOF? */ > + if (eof || bma.got.br_startoff > bno) { > + if (flags & XFS_BMAPI_DELALLOC) { > + /* > + * For the COW fork we can reasonably get a > + * request for converting an extent that races > + * with other threads already having converted > + * part of it, as there converting COW to > + * regular blocks is not protected using the > + * IOLOCK. > + */ > + ASSERT(flags & XFS_BMAPI_COWFORK); > + if (!(flags & XFS_BMAPI_COWFORK)) { > + error = -EIO; > + goto error0; > + } > + } else { > + need_alloc = true; > + } > + } else { > + /* > + * Make sure we only reflink into a hole. > + */ > + ASSERT(!(flags & XFS_BMAPI_REMAP)); > + if (isnullstartblock(bma.got.br_startblock)) > + wasdelay = true; > + } > > /* > * First, deal with the hole before the allocated space > * that we found, if any. > */ > - if (inhole || wasdelay) { > + if (need_alloc || wasdelay) { > bma.eof = eof; > bma.conv = !!(flags & XFS_BMAPI_CONVERT); > bma.wasdel = wasdelay; > diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h > index cecd094..8f805a7 100644 > --- a/fs/xfs/libxfs/xfs_bmap.h > +++ b/fs/xfs/libxfs/xfs_bmap.h > @@ -110,6 +110,9 @@ struct xfs_extent_free_item > /* Map something in the CoW fork. */ > #define XFS_BMAPI_COWFORK 0x200 > > +/* Only convert delalloc space, don't allocate entirely new extents */ > +#define XFS_BMAPI_DELALLOC 0x400 > + > #define XFS_BMAPI_FLAGS \ > { XFS_BMAPI_ENTIRE, "ENTIRE" }, \ > { XFS_BMAPI_METADATA, "METADATA" }, \ XFS_BMAPI_FLAGS needs an entry for XFS_BMAPI_DELALLOC so that tracing continues to work properly. Other than that this patch seems ok to me... --D > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c > index ca137b7..7ee8629 100644 > --- a/fs/xfs/xfs_iomap.c > +++ b/fs/xfs/xfs_iomap.c > @@ -795,7 +795,7 @@ xfs_iomap_write_allocate( > xfs_trans_t *tp; > int nimaps; > int error = 0; > - int flags = 0; > + int flags = XFS_BMAPI_DELALLOC; > int nres; > > if (whichfork == XFS_COW_FORK) > -- > 2.1.4 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c index 44773c9..ceae779 100644 --- a/fs/xfs/libxfs/xfs_bmap.c +++ b/fs/xfs/libxfs/xfs_bmap.c @@ -4514,8 +4514,6 @@ xfs_bmapi_write( int n; /* current extent index */ xfs_fileoff_t obno; /* old block number (offset) */ int whichfork; /* data or attr fork */ - char inhole; /* current location is hole in file */ - char wasdelay; /* old extent was delayed */ #ifdef DEBUG xfs_fileoff_t orig_bno; /* original block number value */ @@ -4603,22 +4601,41 @@ xfs_bmapi_write( bma.firstblock = firstblock; while (bno < end && n < *nmap) { - inhole = eof || bma.got.br_startoff > bno; - wasdelay = !inhole && isnullstartblock(bma.got.br_startblock); + bool need_alloc = false, wasdelay = false; - /* - * Make sure we only reflink into a hole. - */ - if (flags & XFS_BMAPI_REMAP) - ASSERT(inhole); - if (flags & XFS_BMAPI_COWFORK) - ASSERT(!inhole); + /* in hole or beyoned EOF? */ + if (eof || bma.got.br_startoff > bno) { + if (flags & XFS_BMAPI_DELALLOC) { + /* + * For the COW fork we can reasonably get a + * request for converting an extent that races + * with other threads already having converted + * part of it, as there converting COW to + * regular blocks is not protected using the + * IOLOCK. + */ + ASSERT(flags & XFS_BMAPI_COWFORK); + if (!(flags & XFS_BMAPI_COWFORK)) { + error = -EIO; + goto error0; + } + } else { + need_alloc = true; + } + } else { + /* + * Make sure we only reflink into a hole. + */ + ASSERT(!(flags & XFS_BMAPI_REMAP)); + if (isnullstartblock(bma.got.br_startblock)) + wasdelay = true; + } /* * First, deal with the hole before the allocated space * that we found, if any. */ - if (inhole || wasdelay) { + if (need_alloc || wasdelay) { bma.eof = eof; bma.conv = !!(flags & XFS_BMAPI_CONVERT); bma.wasdel = wasdelay; diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h index cecd094..8f805a7 100644 --- a/fs/xfs/libxfs/xfs_bmap.h +++ b/fs/xfs/libxfs/xfs_bmap.h @@ -110,6 +110,9 @@ struct xfs_extent_free_item /* Map something in the CoW fork. */ #define XFS_BMAPI_COWFORK 0x200 +/* Only convert delalloc space, don't allocate entirely new extents */ +#define XFS_BMAPI_DELALLOC 0x400 + #define XFS_BMAPI_FLAGS \ { XFS_BMAPI_ENTIRE, "ENTIRE" }, \ { XFS_BMAPI_METADATA, "METADATA" }, \ diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c index ca137b7..7ee8629 100644 --- a/fs/xfs/xfs_iomap.c +++ b/fs/xfs/xfs_iomap.c @@ -795,7 +795,7 @@ xfs_iomap_write_allocate( xfs_trans_t *tp; int nimaps; int error = 0; - int flags = 0; + int flags = XFS_BMAPI_DELALLOC; int nres; if (whichfork == XFS_COW_FORK)
Due to the way how xfs_iomap_write_allocate tries to convert the whole found extents from delalloc to real space we can run into a race condition with multiple threads doing writes to this same extent. For the non-COW case that is harmless as the only thing that can happen is that we call xfs_bmapi_write on an extent that has already been converted to a real allocation. For COW writes where we move the extent from the COW to the data fork after I/O completion the race is, however, not quite as harmless. In the worst case we are now calling xfs_bmapi_write on a region that contains hole in the COW work, which will trip up an assert in debug builds or lead to file system corruption in non-debug builds. This seems to be reproducible with workloads of small O_DSYNC write, although so far I've not managed to come up with a with an isolated reproducer. The fix for the issue is relatively simple: tell xfs_bmapi_write that we are only asked to convert delayed allocations and skip holes in that case. Signed-off-by: Christoph Hellwig <hch@lst.de> --- fs/xfs/libxfs/xfs_bmap.c | 41 +++++++++++++++++++++++++++++------------ fs/xfs/libxfs/xfs_bmap.h | 3 +++ fs/xfs/xfs_iomap.c | 2 +- 3 files changed, 33 insertions(+), 13 deletions(-)