Re: [PATCH RFC 5/7] fs: iomap: buffered atomic write support

[Matthew Wilcox]

On Mon, Apr 22, 2024 at 02:39:21PM +0000, John Garry wrote:
> Add special handling of PG_atomic flag to iomap buffered write path.
> 
> To flag an iomap iter for an atomic write, set IOMAP_ATOMIC.
> 
> For a folio associated with a write which has IOMAP_ATOMIC set, set
> PG_atomic.
> 
> Otherwise, when IOMAP_ATOMIC is unset, clear PG_atomic.
> 
> This means that for an "atomic" folio which has not been written back, it
> loses it "atomicity". So if userspace issues a write with RWF_ATOMIC set
> and another write with RWF_ATOMIC unset and which fully or partially
> overwrites that same region as the first write, that folio is not written
> back atomically. For such a scenario to occur, it would be considered a
> userspace usage error.
> 
> To ensure that a buffered atomic write is written back atomically when
> the write syscall returns, RWF_SYNC or similar needs to be used (in
> conjunction with RWF_ATOMIC).
> 
> As a safety check, when getting a folio for an atomic write in
> iomap_get_folio(), ensure that the length matches the inode mapping folio
> order-limit.
> 
> Only a single BIO should ever be submitted for an atomic write. So modify
> iomap_add_to_ioend() to ensure that we don't try to write back an atomic
> folio as part of a larger mixed-atomicity BIO.
> 
> In iomap_alloc_ioend(), handle an atomic write by setting REQ_ATOMIC for
> the allocated BIO.
> 
> When a folio is written back, again clear PG_atomic, as it is no longer
> required. I assume it will not be needlessly written back a second time...

I'm not taking a position on the mechanism yet; need to think about it
some more.  But there's a hole here I also don't have a solution to,
so we can all start thinking about it.

In iomap_write_iter(), we call copy_folio_from_iter_atomic().  Through no
fault of the application, if the range crosses a page boundary, we might
partially copy the bytes from the first page, then take a page fault on
the second page, hence doing a short write into the folio.  And there's
nothing preventing writeback from writing back a partially copied folio.

Now, if it's not dirty, then it can't be written back.  So if we're
doing an atomic write, we could clear the dirty bit after calling
iomap_write_begin() (given the usage scenarios we've discussed, it should
always be clear ...)

We need to prevent the "fall back to a short copy" logic in
iomap_write_iter() as well.  But then we also need to make sure we don't
get stuck in a loop, so maybe go three times around, and if it's still
not readable as a chunk, -EFAULT?