From: Matthew Wilcox <willy@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
ocfs2-devel@oss.oracle.com, linux-xfs@vger.kernel.org
Subject: Re: [PATCH v6 11/19] btrfs: Convert from readpages to readahead
Date: Tue, 18 Feb 2020 13:12:28 -0800 [thread overview]
Message-ID: <20200218211228.GF24185@bombadil.infradead.org> (raw)
In-Reply-To: <20200218065758.GQ10776@dread.disaster.area>
On Tue, Feb 18, 2020 at 05:57:58PM +1100, Dave Chinner wrote:
> On Mon, Feb 17, 2020 at 10:45:59AM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >
> > Use the new readahead operation in btrfs. Add a
> > readahead_for_each_batch() iterator to optimise the loop in the XArray.
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> > fs/btrfs/extent_io.c | 46 +++++++++++++----------------------------
> > fs/btrfs/extent_io.h | 3 +--
> > fs/btrfs/inode.c | 16 +++++++-------
> > include/linux/pagemap.h | 27 ++++++++++++++++++++++++
> > 4 files changed, 49 insertions(+), 43 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index c0f202741e09..e97a6acd6f5d 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -4278,52 +4278,34 @@ int extent_writepages(struct address_space *mapping,
> > return ret;
> > }
> >
> > -int extent_readpages(struct address_space *mapping, struct list_head *pages,
> > - unsigned nr_pages)
> > +void extent_readahead(struct readahead_control *rac)
> > {
> > struct bio *bio = NULL;
> > unsigned long bio_flags = 0;
> > struct page *pagepool[16];
> > struct extent_map *em_cached = NULL;
> > - struct extent_io_tree *tree = &BTRFS_I(mapping->host)->io_tree;
> > - int nr = 0;
> > + struct extent_io_tree *tree = &BTRFS_I(rac->mapping->host)->io_tree;
> > u64 prev_em_start = (u64)-1;
> > + int nr;
> >
> > - while (!list_empty(pages)) {
> > - u64 contig_end = 0;
> > -
> > - for (nr = 0; nr < ARRAY_SIZE(pagepool) && !list_empty(pages);) {
> > - struct page *page = lru_to_page(pages);
> > -
> > - prefetchw(&page->flags);
> > - list_del(&page->lru);
> > - if (add_to_page_cache_lru(page, mapping, page->index,
> > - readahead_gfp_mask(mapping))) {
> > - put_page(page);
> > - break;
> > - }
> > -
> > - pagepool[nr++] = page;
> > - contig_end = page_offset(page) + PAGE_SIZE - 1;
> > - }
> > + readahead_for_each_batch(rac, pagepool, ARRAY_SIZE(pagepool), nr) {
> > + u64 contig_start = page_offset(pagepool[0]);
> > + u64 contig_end = page_offset(pagepool[nr - 1]) + PAGE_SIZE - 1;
>
> So this assumes a contiguous page range is returned, right?
Yes. That's documented in the readahead API and is the behaviour of
the code. I mean, btrfs asserts it's true while most of the rest of
the kernel is indifferent to it, but it's the documented and actual
behaviour.
> >
> > - if (nr) {
> > - u64 contig_start = page_offset(pagepool[0]);
> > + ASSERT(contig_start + nr * PAGE_SIZE - 1 == contig_end);
>
> Ok, yes it does. :)
>
> I don't see how readahead_for_each_batch() guarantees that, though.
I ... don't see how it doesn't? We start at rac->_start and iterate
through the consecutive pages in the page cache. readahead_for_each_batch()
does assume that __do_page_cache_readahead() has its current behaviour
of putting the pages in the page cache in order, and kicks off a new
call to ->readahead() every time it has to skip an index for whatever
reason (eg page already in page cache).
> > - if (bio)
> > - return submit_one_bio(bio, 0, bio_flags);
> > - return 0;
> > + if (bio) {
> > + if (submit_one_bio(bio, 0, bio_flags))
> > + return;
> > + }
> > }
>
> Shouldn't that just be
>
> if (bio)
> submit_one_bio(bio, 0, bio_flags);
It should, but some overzealous person decided to mark submit_one_bio()
as __must_check, so I have to work around that.
> > +static inline unsigned int readahead_page_batch(struct readahead_control *rac,
> > + struct page **array, unsigned int size)
> > +{
> > + unsigned int batch = 0;
>
> Confusing when put alongside rac->_batch_count counting the number
> of pages in the batch, and "batch" being the index into the page
> array, and they aren't the same counts....
Yes. Renamed to 'i'.
> > + XA_STATE(xas, &rac->mapping->i_pages, rac->_start);
> > + struct page *page;
> > +
> > + rac->_batch_count = 0;
> > + xas_for_each(&xas, page, rac->_start + rac->_nr_pages - 1) {
>
> That just iterates pages in the start,end doesn't it? What
> guarantees that this fills the array with a contiguous page range?
The behaviour of __do_page_cache_readahead(). Dave Howells also has a
usecase for xas_for_each_contig(), so I'm going to add that soon.
> > + VM_BUG_ON_PAGE(!PageLocked(page), page);
> > + VM_BUG_ON_PAGE(PageTail(page), page);
> > + array[batch++] = page;
> > + rac->_batch_count += hpage_nr_pages(page);
> > + if (PageHead(page))
> > + xas_set(&xas, rac->_start + rac->_batch_count);
>
> What on earth does this do? Comments please!
/*
* The page cache isn't using multi-index entries yet,
* so xas_for_each() won't do the right thing for
* large pages. This can be removed once the page cache
* is converted.
*/
> > +
> > + if (batch == size)
> > + break;
> > + }
> > +
> > + return batch;
> > +}
>
> Seems a bit big for an inline function.
It's only called by btrfs at the moment. If it gets more than one caller,
then sure, let's move it out of line.
> > +
> > +#define readahead_for_each_batch(rac, array, size, nr) \
> > + for (; (nr = readahead_page_batch(rac, array, size)); \
> > + readahead_next(rac))
>
> I had to go look at the caller to work out what "size" refered to
> here.
>
> This is complex enough that it needs proper API documentation.
How about just:
-#define readahead_for_each_batch(rac, array, size, nr) \
- for (; (nr = readahead_page_batch(rac, array, size)); \
+#define readahead_for_each_batch(rac, array, array_sz, nr) \
+ for (; (nr = readahead_page_batch(rac, array, array_sz)); \
(corresponding rename in readahead_page_batch). I mean, we could also
do:
#define readahead_for_each_batch(rac, array, nr) \
for (; (nr = readahead_page_batch(rac, array, ARRAY_SIZE(array)); \
readahead_next(rac))
making it less flexible, but easier to use.
next prev parent reply other threads:[~2020-02-18 21:12 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-17 18:45 [PATCH v6 00/19] Change readahead API Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 01/19] mm: Return void from various readahead functions Matthew Wilcox
2020-02-18 4:47 ` Dave Chinner
2020-02-18 21:05 ` John Hubbard
2020-02-18 21:21 ` Matthew Wilcox
2020-02-18 21:52 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 02/19] mm: Ignore return value of ->readpages Matthew Wilcox
2020-02-18 4:48 ` Dave Chinner
2020-02-18 21:33 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 03/19] mm: Use readahead_control to pass arguments Matthew Wilcox
2020-02-18 5:03 ` Dave Chinner
2020-02-18 13:56 ` Matthew Wilcox
2020-02-18 22:46 ` Dave Chinner
2020-02-18 22:52 ` Matthew Wilcox
2020-02-18 22:22 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 04/19] mm: Rearrange readahead loop Matthew Wilcox
2020-02-18 5:08 ` Dave Chinner
2020-02-18 13:57 ` Matthew Wilcox
2020-02-18 22:48 ` Dave Chinner
2020-02-18 22:33 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 04/16] mm: Tweak readahead loop slightly Matthew Wilcox
2020-02-18 22:57 ` John Hubbard
2020-02-18 23:00 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 05/16] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 05/19] mm: Remove 'page_offset' from readahead loop Matthew Wilcox
2020-02-18 5:14 ` Dave Chinner
2020-02-18 23:08 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 06/16] mm: Add readahead address space operation Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 06/19] mm: rename readahead loop variable to 'i' Matthew Wilcox
2020-02-18 5:33 ` Dave Chinner
2020-02-18 23:11 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 07/16] mm: Add page_cache_readahead_limit Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 07/19] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-02-18 6:14 ` Dave Chinner
2020-02-18 15:42 ` Matthew Wilcox
2020-02-19 0:59 ` Dave Chinner
2020-02-19 0:01 ` John Hubbard
2020-02-19 1:02 ` Matthew Wilcox
2020-02-19 1:13 ` John Hubbard
2020-02-19 3:24 ` John Hubbard
2020-02-19 14:41 ` Matthew Wilcox
2020-02-19 14:52 ` Christoph Hellwig
2020-02-19 15:01 ` Matthew Wilcox
2020-02-19 20:24 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 08/16] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 08/19] mm: Add readahead address space operation Matthew Wilcox
2020-02-18 6:21 ` Dave Chinner
2020-02-18 16:10 ` Matthew Wilcox
2020-02-19 1:04 ` Dave Chinner
2020-02-19 0:12 ` John Hubbard
2020-02-19 3:10 ` Eric Biggers
2020-02-19 3:35 ` Eric Biggers
2020-02-19 16:52 ` Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 09/16] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 09/19] mm: Add page_cache_readahead_limit Matthew Wilcox
2020-02-18 6:31 ` Dave Chinner
2020-02-18 19:54 ` Matthew Wilcox
2020-02-19 1:08 ` Dave Chinner
2020-02-19 1:32 ` John Hubbard
2020-02-19 2:23 ` Matthew Wilcox
2020-02-19 2:46 ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 10/16] erofs: Convert uncompressed files from readpages to readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 10/19] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-02-18 1:51 ` [Ocfs2-devel] " Joseph Qi
2020-02-18 6:37 ` Dave Chinner
2020-02-19 2:48 ` John Hubbard
2020-02-19 3:28 ` Eric Biggers
2020-02-19 3:47 ` Matthew Wilcox
2020-02-19 3:55 ` Eric Biggers
2020-02-17 18:45 ` [PATCH v6 11/19] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-02-18 6:57 ` Dave Chinner
2020-02-18 21:12 ` Matthew Wilcox [this message]
2020-02-19 1:23 ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 11/16] erofs: Convert compressed files " Matthew Wilcox
2020-02-19 2:34 ` Gao Xiang
2020-02-17 18:46 ` [PATCH v6 12/19] erofs: Convert uncompressed " Matthew Wilcox
2020-02-19 2:39 ` Gao Xiang
2020-02-19 3:04 ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 12/16] ext4: Convert " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 13/19] erofs: Convert compressed files " Matthew Wilcox
2020-02-19 3:08 ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 13/16] f2fs: Convert " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 14/19] ext4: " Matthew Wilcox
2020-02-19 3:16 ` Dave Chinner
2020-02-19 3:29 ` Eric Biggers
2020-02-17 18:46 ` [PATCH v6 14/16] fuse: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 15/19] f2fs: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 15/16] iomap: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 16/19] fuse: " Matthew Wilcox
2020-02-19 3:22 ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 16/16] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor Matthew Wilcox
2020-02-19 3:17 ` John Hubbard
2020-02-19 5:35 ` Matthew Wilcox
2020-02-19 3:29 ` Dave Chinner
2020-02-19 6:04 ` Matthew Wilcox
2020-02-19 6:40 ` Dave Chinner
2020-02-19 17:06 ` Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 18/19] iomap: Convert from readpages to readahead Matthew Wilcox
2020-02-19 3:40 ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 19/19] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-02-19 3:43 ` Dave Chinner
2020-02-19 5:22 ` Matthew Wilcox
2020-02-17 18:48 ` [PATCH v6 00/19] Change readahead API Matthew Wilcox
2020-02-18 4:56 ` Dave Chinner
2020-02-18 13:42 ` Matthew Wilcox
2020-02-18 21:26 ` Dave Chinner
2020-02-19 3:45 ` Dave Chinner
2020-02-19 3:48 ` Matthew Wilcox
2020-02-19 3:57 ` Dave Chinner
2020-02-18 20:49 ` John Hubbard
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