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We can't have atomic writes to get split. >>>>> This patch is just lifting the restriction of iomap to allow more than >>>>> blocksize but the mapped length should still meet iter->len, as >>>>> otherwise the writes can get split. >>>> Sure, I get this. But I wonder why would we be getting multiple >>>> mappings? Why cannot the FS always provide a single mapping? >>> FS can decide to split the mappings when it couldn't allocate a single >>> large mapping of the requested length. Could be due to - >>> - already allocated extent followed by EOF, >>> - already allocated extent followed by a hole >>> - already mapped extent followed by an extent of different type (e.g. written followed by unwritten or unwritten followed by written) >> >> This is the sort of scenario which I am concerned with. This issue has >> been discussed at length for XFS forcealign support for atomic writes. > > extsize and forcealign is being worked for ext4 as well where we can > add such support, sure. > >> >> So far, the user can atomic write a single FS block regardless of >> whether the extent in which it would be part of is in written or >> unwritten state. >> >> Now the rule will be to write multiple FS blocks atomically, all blocks >> need to be in same written or unwritten state. > > FS needs to ensure that the writes does not get torned. So for whatever reason > FS splits the mapping then we need to return an -EINVAL error to not > allow such writes to get torned. This patch just does that. > > But I get your point. More below. > >> >> This oddity at least needs to be documented. > > Got it. Yes, we can do that. > >> >> Better yet would be to not have this restriction. >> > > I haven't thought of a clever way where we don't have to zero out the > rest of the unwritten mapping. With ext4 bigalloc since the entire > cluster is anyway reserved - I was thinking if we can come up with a > clever way for doing atomic writes to the entire user requested size w/o > zeroing out. This following was main method which was being attempted: https://lore.kernel.org/linux-fsdevel/20240429174746.2132161-15-john.g.garry@oracle.com/ There were other ideas in different versions of the forcelign/xfs block atomic writes series. > > Zeroing out the other unwritten extent is also a cost penalty to the > user anyways. Sure, unless we have a special inode flag to say "pre-zero the extent". > So user will anyway will have to be made aware of not to > attempt writes of fashion which can cause them such penalties. > > As patch-6 mentions this is a base support for bs = ps systems for > enabling atomic writes using bigalloc. For now we return -EINVAL when we > can't allocate a continuous user requested mapping which means it won't > support operations of types 8k followed by 16k. > That's my least-preferred option. I think better would be reject atomic writes that cover unwritten extents always - but that boat is about to sail... Thanks, John