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[49.186.4.237]) by smtp.gmail.com with ESMTPSA id q25-20020a62ae19000000b005a84ef49c63sm6574866pff.214.2023.02.28.17.04.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Feb 2023 17:04:19 -0800 (PST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1pXAtF-003KxZ-4K; Wed, 01 Mar 2023 12:04:17 +1100 Date: Wed, 1 Mar 2023 12:04:17 +1100 From: Dave Chinner To: "Darrick J. Wong" Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH] xfs: fix off-by-one-block in xfs_discard_folio() Message-ID: <20230301010417.GE360264@dread.disaster.area> References: <20230301001706.1315973-1-david@fromorbit.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Feb 28, 2023 at 04:47:01PM -0800, Darrick J. Wong wrote: > On Wed, Mar 01, 2023 at 11:17:06AM +1100, Dave Chinner wrote: > > From: Dave Chinner > > > > The recent writeback corruption fixes changed the code in > > xfs_discard_folio() to calculate a byte range to for punching > > delalloc extents. A mistake was made in using round_up(pos) for the > > end offset, because when pos points at the first byte of a block, it > > does not get rounded up to point to the end byte of the block. hence > > the punch range is short, and this leads to unexpected behaviour in > > certain cases in xfs_bmap_punch_delalloc_range. > > > > e.g. pos = 0 means we call xfs_bmap_punch_delalloc_range(0,0), so > > there is no previous extent and it rounds up the punch to the end of > > the delalloc extent it found at offset 0, not the end of the range > > given to xfs_bmap_punch_delalloc_range(). > > > > Fix this by handling the zero block offset case correctly. > > > > Fixes: 7348b322332d ("xfs: xfs_bmap_punch_delalloc_range() should take a byte range") > > Reported-by: Pengfei Xu > > Found-by: Brian Foster > > Signed-off-by: Dave Chinner > > --- > > fs/xfs/xfs_aops.c | 14 ++++++++++++-- > > 1 file changed, 12 insertions(+), 2 deletions(-) > > > > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c > > index 41734202796f..429f63cfd7d4 100644 > > --- a/fs/xfs/xfs_aops.c > > +++ b/fs/xfs/xfs_aops.c > > @@ -466,6 +466,7 @@ xfs_discard_folio( > > { > > struct xfs_inode *ip = XFS_I(folio->mapping->host); > > struct xfs_mount *mp = ip->i_mount; > > + xfs_off_t end_off; > > int error; > > > > if (xfs_is_shutdown(mp)) > > @@ -475,8 +476,17 @@ xfs_discard_folio( > > "page discard on page "PTR_FMT", inode 0x%llx, pos %llu.", > > folio, ip->i_ino, pos); > > > > - error = xfs_bmap_punch_delalloc_range(ip, pos, > > - round_up(pos, folio_size(folio))); > > + /* > > + * Need to be careful with the case where the pos passed in points to > > + * the first byte of the folio - rounding up won't change the value, > > + * but in all cases here we need to end offset to point to the start > > + * of the next folio. > > + */ > > + if (pos == folio_pos(folio)) > > + end_off = pos + folio_size(folio); > > + else > > + end_off = round_up(pos, folio_size(folio)); > > Can this construct be simplified to: > > end_off = round_up(pos + 1, folio_size(folio)); I thought about that first, but I really, really dislike sprinkling magic "+ 1" corrections into the code to address non-obvious unexplained off-by-one problems. > If pos is the first byte of the folio, it'll round end_off to the start > of the next folio. If pos is (somehow) the last byte of the folio, the > first argument to round_up is already the first byte of the next folio, > and rounding won't change it. Yup, and that's exactly the problem I had with doing this - it relies on the implicit behaviour that by moving last byte of a block to the first byte of the next block, round_up() won't change the end offset. i.e. the correct functioning of the code is just as non-obvious with a magic "+ 1" as the incorrect functioning was without it. Look at it this way: I didn't realise it was wrong when I wrote the code, and I couldn't find the bug round_up() introduced when reading the code even after the problem had been bisected to this exact change. The code I wrote is bad, and adding a magic "+ 1" to fix the bug doesn't make the code any better. Given this is a slow path, so I see no point in optimising the code for efficiency. IMO, clarity of the logic and calculation being made is far more important - obviously correct logic is better than relying on the effect of a magic "+ 1" on some other function to acheive the same thing.... Cheers, Dave. -- Dave Chinner david@fromorbit.com