Re: [PATCH 03/12] filemap: update ki_pos in generic_perform_write

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On Sun, Aug 27, 2023 at 08:41:22PM +0100, Al Viro wrote:
> That part is somewhat fishy - there's a case where you return a positive value
> and advance ->ki_pos by more than that amount.  I really wonder if all callers
> of ->write_iter() are OK with that.  Consider e.g. this:

This should not exist in the latest version merged by Jens.  Can you
check if you still  see issues in the version in the block tree or

> Suppose ->write_iter() ends up doing returning a positive value smaller than
> the increment of kiocb.ki_pos.  What do we get?  ret is positive, so
> kiocb.ki_pos gets copied into *ppos, which is ksys_write's pos and there
> we copy it into file->f_pos.
> Is it really OK to have write() return 4096 and advance the file position
> by 16K?  AFAICS, userland wouldn't get any indication of something
> odd going on - just a short write to a regular file, with followup write
> of remaining 12K getting quietly written in the range 16K..28K.
> I don't remember what POSIX says about that, but it would qualify as
> nasty surprise for any userland program - sure, one can check fsync()
> results before closing the sucker and see if everything looks fine,
> but the way it's usually discussed could easily lead to assumption that
> (synchronous) O_DIRECT writes would not be affected by anything of that
> sort.

ki_pos should always be updated by the write return value.  Everything
else is a bug.

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