From: Matthew Wilcox <willy@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
Wang Yugui <wangyugui@e16-tech.com>,
Christoph Hellwig <hch@infradead.org>,
"Darrick J . Wong" <djwong@kernel.org>
Subject: Re: [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios
Date: Tue, 13 Jun 2023 03:00:14 +0100 [thread overview]
Message-ID: <ZIfNrnUsJbcWGSD8@casper.infradead.org> (raw)
In-Reply-To: <ZIfGpWYNA1yd5K/l@dread.disaster.area>
On Tue, Jun 13, 2023 at 11:30:13AM +1000, Dave Chinner wrote:
> On Tue, Jun 13, 2023 at 01:42:51AM +0100, Matthew Wilcox wrote:
> > On Tue, Jun 13, 2023 at 08:49:05AM +1000, Dave Chinner wrote:
> > > On Mon, Jun 12, 2023 at 09:39:08PM +0100, Matthew Wilcox (Oracle) wrote:
> > > > Allow callers of __filemap_get_folio() to specify a preferred folio
> > > > order in the FGP flags. This is only honoured in the FGP_CREATE path;
> > > > if there is already a folio in the page cache that covers the index,
> > > > we will return it, no matter what its order is. No create-around is
> > > > attempted; we will only create folios which start at the specified index.
> > > > Unmodified callers will continue to allocate order 0 folios.
> > > .....
> > > > - /* Init accessed so avoid atomic mark_page_accessed later */
> > > > - if (fgp_flags & FGP_ACCESSED)
> > > > - __folio_set_referenced(folio);
> > > > + if (!mapping_large_folio_support(mapping))
> > > > + order = 0;
> > > > + if (order > MAX_PAGECACHE_ORDER)
> > > > + order = MAX_PAGECACHE_ORDER;
> > > > + /* If we're not aligned, allocate a smaller folio */
> > > > + if (index & ((1UL << order) - 1))
> > > > + order = __ffs(index);
> > >
> > > If I read this right, if we pass in an unaligned index, we won't get
> > > the size of the folio we ask for?
> >
> > Right. That's implied by (but perhaps not obvious from) the changelog.
> > Folios are always naturally aligned in the file, so an order-4 folio
> > has to start at a multiple of 16. If the index you pass in is not
> > a multiple of 16, we can't create an order-4 folio without starting
> > at an earlier index.
> >
> > For a 4kB block size filesystem, that's what we want. Applications
> > _generally_ don't write backwards, so creating an order-4 folio is just
> > wasting memory.
> >
> > > e.g. if we want an order-4 folio (64kB) because we have a 64kB block
> > > size in the filesystem, then we have to pass in an index that
> > > order-4 aligned, yes?
> > >
> > > I ask this, because the later iomap code that asks for large folios
> > > only passes in "pos >> PAGE_SHIFT" so it looks to me like it won't
> > > allocate large folios for anything other than large folio aligned
> > > writes, even if we need them.
> > >
> > > What am I missing?
> >
> > Perhaps what you're missing is that this isn't trying to solve the
> > problem of supporting a bs > ps filesystem?
>
> No, that's not what I'm asking about. I know there's other changes
> needed to enforce minimum folio size/alignment for bs > ps.
OK. Bringing up the 64kB block size filesystem confused me.
> What I'm asking about is when someone does a 16kB write at offset
> 12kB, they won't get a large folio allocated at all, right? Even
> though the write is large enough to enable it?
Right.
> Indeed, if we do a 1MB write at offset 4KB, we'll get 4kB at 4KB, 8KB
> and 12kB (because we can't do order-1 folios), then order-2 at 16KB,
> order-3 at 32kB, and so on until we hit offset 1MB where we will do
> an order-0 folio allocation again (because the remaining length is
> 4KB). The next 1MB write will then follow the same pattern, right?
Yes. Assuming we get another write ...
> I think this ends up being sub-optimal and fairly non-obvious
> non-obvious behaviour from the iomap side of the fence which is
> clearly asking for high-order folios to be allocated. i.e. a small
> amount of allocate-around to naturally align large folios when the
> page cache is otherwise empty would make a big difference to the
> efficiency of non-large-folio-aligned sequential writes...
At this point we're arguing about what I/O pattern to optimise for.
I'm going for a "do no harm" approach where we only allocate exactly as
much memory as we did before. You're advocating for a
higher-risk/higher-reward approach.
I'd prefer the low-risk approach for now; we can change it later!
I'd like to see some amount of per-fd write history (as we have per-fd
readahead history) to decide whether to allocate large folios ahead of
the current write position. As with readahead, I'd like to see that even
doing single-byte writes can result in the allocation of large folios,
as long as the app has done enough of them.
next prev parent reply other threads:[~2023-06-13 2:00 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-12 20:39 [PATCH v3 0/8] Create large folios in iomap buffered write path Matthew Wilcox (Oracle)
2023-06-12 20:39 ` [PATCH v3 1/8] iov_iter: Handle compound highmem pages in copy_page_from_iter_atomic() Matthew Wilcox (Oracle)
2023-06-13 4:52 ` Christoph Hellwig
2023-07-10 3:36 ` Matthew Wilcox
2023-06-12 20:39 ` [PATCH v3 2/8] iomap: Remove large folio handling in iomap_invalidate_folio() Matthew Wilcox (Oracle)
2023-06-12 20:39 ` [PATCH v3 3/8] doc: Correct the description of ->release_folio Matthew Wilcox (Oracle)
2023-06-13 4:53 ` Christoph Hellwig
2023-06-12 20:39 ` [PATCH v3 4/8] iomap: Remove unnecessary test from iomap_release_folio() Matthew Wilcox (Oracle)
2023-06-13 4:53 ` Christoph Hellwig
2023-06-13 16:19 ` Matthew Wilcox
2023-06-12 20:39 ` [PATCH v3 5/8] filemap: Add fgf_t typedef Matthew Wilcox (Oracle)
2023-06-13 4:53 ` Christoph Hellwig
2023-06-12 20:39 ` [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios Matthew Wilcox (Oracle)
2023-06-12 22:49 ` Dave Chinner
2023-06-13 0:42 ` Matthew Wilcox
2023-06-13 1:30 ` Dave Chinner
2023-06-13 2:00 ` Matthew Wilcox [this message]
2023-06-13 7:54 ` Dave Chinner
2023-06-13 13:34 ` Matthew Wilcox
2023-06-16 17:45 ` Matthew Wilcox
2023-06-16 22:40 ` Dave Chinner
2023-06-13 4:56 ` Christoph Hellwig
2023-06-12 20:39 ` [PATCH v3 7/8] iomap: Create large folios in the buffered write path Matthew Wilcox (Oracle)
2023-06-13 4:56 ` Christoph Hellwig
2023-06-12 20:39 ` [PATCH v3 8/8] iomap: Copy larger chunks from userspace Matthew Wilcox (Oracle)
2023-06-13 4:58 ` Christoph Hellwig
2023-06-13 19:43 ` Matthew Wilcox
2023-07-10 3:45 ` Matthew Wilcox
2023-06-17 7:13 ` Ritesh Harjani
2023-06-19 17:09 ` Matthew Wilcox
2023-07-10 3:57 ` Matthew Wilcox
2023-06-21 12:03 ` [PATCH v3 0/8] Create large folios in iomap buffered write path Wang Yugui
2023-07-10 3:55 ` Matthew Wilcox
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