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Wong" Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org, willy@infradead.org Subject: Re: [PATCH v3 7/7] xfs: error tag to force zeroing on debug kernels Message-ID: References: <20250714204122.349582-1-bfoster@redhat.com> <20250714204122.349582-8-bfoster@redhat.com> <20250715052444.GP2672049@frogsfrogsfrogs> <20250715143041.GN2672029@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250715143041.GN2672029@frogsfrogsfrogs> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On Tue, Jul 15, 2025 at 07:30:41AM -0700, Darrick J. Wong wrote: > On Tue, Jul 15, 2025 at 08:39:03AM -0400, Brian Foster wrote: > > On Mon, Jul 14, 2025 at 10:24:44PM -0700, Darrick J. Wong wrote: > > > On Mon, Jul 14, 2025 at 04:41:22PM -0400, Brian Foster wrote: > > > > iomap_zero_range() has to cover various corner cases that are > > > > difficult to test on production kernels because it is used in fairly > > > > limited use cases. For example, it is currently only used by XFS and > > > > mostly only in partial block zeroing cases. > > > > > > > > While it's possible to test most of these functional cases, we can > > > > provide more robust test coverage by co-opting fallocate zero range > > > > to invoke zeroing of the entire range instead of the more efficient > > > > block punch/allocate sequence. Add an errortag to occasionally > > > > invoke forced zeroing. > > > > > > > > Signed-off-by: Brian Foster > > > > Reviewed-by: Christoph Hellwig > > > > --- > > > > fs/xfs/libxfs/xfs_errortag.h | 4 +++- > > > > fs/xfs/xfs_error.c | 3 +++ > > > > fs/xfs/xfs_file.c | 26 ++++++++++++++++++++------ > > > > 3 files changed, 26 insertions(+), 7 deletions(-) > > > > > > ... > > > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c > > > > index 0b41b18debf3..c865f9555b77 100644 > > > > --- a/fs/xfs/xfs_file.c > > > > +++ b/fs/xfs/xfs_file.c > > > > @@ -27,6 +27,8 @@ > > > > #include "xfs_file.h" > > > > #include "xfs_aops.h" > > > > #include "xfs_zone_alloc.h" > > > > +#include "xfs_error.h" > > > > +#include "xfs_errortag.h" > > > > > > > > #include > > > > #include > > > > @@ -1269,13 +1271,25 @@ xfs_falloc_zero_range( > > > > if (error) > > > > return error; > > > > > > > > - error = xfs_free_file_space(XFS_I(inode), offset, len, ac); > > > > - if (error) > > > > - return error; > > > > + /* > > > > + * Zero range implements a full zeroing mechanism but is only used in > > > > + * limited situations. It is more efficient to allocate unwritten > > > > + * extents than to perform zeroing here, so use an errortag to randomly > > > > + * force zeroing on DEBUG kernels for added test coverage. > > > > + */ > > > > + if (XFS_TEST_ERROR(false, XFS_I(inode)->i_mount, > > > > + XFS_ERRTAG_FORCE_ZERO_RANGE)) { > > > > + error = xfs_zero_range(XFS_I(inode), offset, len, ac, NULL); > > > > > > Isn't this basically the ultra slow version fallback version of > > > FALLOC_FL_WRITE_ZEROES ? > > > > > > > ~/linux$ git grep FALLOC_FL_WRITE_ZEROES > > ~/linux$ > > > > IIRC write zeroes is intended to expose fast hardware (physical) zeroing > > (i.e. zeroed written extents)..? If so, I suppose you could consider > > this a fallback of sorts. I'm not sure what write zeroes is expected to > > do in the unwritten extent case, whereas iomap zero range is happy to > > skip those mappings unless they're already dirty in pagecache. > > Sorry, forgot that they weren't wiring anything up in xfs so it never > showed up here: > https://lore.kernel.org/linux-fsdevel/20250619111806.3546162-1-yi.zhang@huaweicloud.com/ > > Basically they want to avoid the unwritten extent conversion overhead by > providing a way to preallocate written zeroed extents and sending magic > commands to hardware that unmaps LBAs in such a way that rereads return > zero. > Ack.. I'd seen that before, but hadn't looked too closely and wasn't sure what the status was. > > Regardless, the purpose of this patch is not to add support for physical > > zeroing, but rather to increase test coverage for the additional code on > > debug kernels because the production use case tends to be more limited. > > This could easily be moved/applied to write zeroes if it makes sense in > > the future and test infra grows support for it. > > On second look, I don't think the new fallocate flag allows for > letting the kernel do pagecache zeroing + flush. Admittedly that would > be beside the point (and userspaces already do that anyway). > Ok. Thanks for the reviews. Brian > Anyway enough mumbling from me, > Reviewed-by: "Darrick J. Wong" > > --D > > > Brian > > > > > --D > > > > > > > + } else { > > > > + error = xfs_free_file_space(XFS_I(inode), offset, len, ac); > > > > + if (error) > > > > + return error; > > > > > > > > - len = round_up(offset + len, blksize) - round_down(offset, blksize); > > > > - offset = round_down(offset, blksize); > > > > - error = xfs_alloc_file_space(XFS_I(inode), offset, len); > > > > + len = round_up(offset + len, blksize) - > > > > + round_down(offset, blksize); > > > > + offset = round_down(offset, blksize); > > > > + error = xfs_alloc_file_space(XFS_I(inode), offset, len); > > > > + } > > > > if (error) > > > > return error; > > > > return xfs_falloc_setsize(file, new_size); > > > > -- > > > > 2.50.0 > > > > > > > > > > > > > > > >