From: Matthew Wilcox <willy@infradead.org>
To: John Garry <john.g.garry@oracle.com>
Cc: axboe@kernel.dk, brauner@kernel.org, djwong@kernel.org,
viro@zeniv.linux.org.uk, jack@suse.cz, akpm@linux-foundation.org,
dchinner@redhat.com, tytso@mit.edu, hch@lst.de,
martin.petersen@oracle.com, nilay@linux.ibm.com,
ritesh.list@gmail.com, mcgrof@kernel.org,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-mm@kvack.org, ojaswin@linux.ibm.com, p.raghav@samsung.com,
jbongio@google.com, okiselev@amazon.com
Subject: Re: [PATCH RFC 5/7] fs: iomap: buffered atomic write support
Date: Mon, 22 Apr 2024 16:03:56 +0100 [thread overview]
Message-ID: <ZiZ8XGZz46D3PRKr@casper.infradead.org> (raw)
In-Reply-To: <20240422143923.3927601-6-john.g.garry@oracle.com>
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?
next prev parent reply other threads:[~2024-04-22 15:04 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-22 14:39 [PATCH RFC 0/7] buffered block atomic writes John Garry
2024-04-22 14:39 ` [PATCH RFC 1/7] fs: Rename STATX{_ATTR}_WRITE_ATOMIC -> STATX{_ATTR}_WRITE_ATOMIC_DIO John Garry
2024-04-22 14:39 ` [PATCH RFC 2/7] filemap: Change mapping_set_folio_min_order() -> mapping_set_folio_orders() John Garry
2024-04-25 14:47 ` Pankaj Raghav (Samsung)
2024-04-26 8:02 ` John Garry
2024-04-22 14:39 ` [PATCH RFC 3/7] mm: Add PG_atomic John Garry
2024-04-22 14:39 ` [PATCH RFC 4/7] fs: Add initial buffered atomic write support info to statx John Garry
2024-04-22 14:39 ` [PATCH RFC 5/7] fs: iomap: buffered atomic write support John Garry
2024-04-22 15:03 ` Matthew Wilcox [this message]
2024-04-22 16:02 ` John Garry
2024-04-22 14:39 ` [PATCH RFC 6/7] fs: xfs: buffered atomic writes statx support John Garry
2024-04-22 14:39 ` [PATCH RFC 7/7] fs: xfs: Enable buffered atomic writes John Garry
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