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Message-ID: <dhfhfcymhfdtqnwof4cqfhplgo24ho3kon3e7lrqwwgz26ehqr@g6h3g3yub4w4>
Date: Thu, 12 Jun 2025 13:37:05 +0200
From: Jan Kara <jack@...e.cz>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, David Hildenbrand <david@...hat.com>,
Dave Chinner <david@...morbit.com>, Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, Kalesh Singh <kaleshsingh@...gle.com>, Zi Yan <ziy@...dia.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH v5 4/5] mm/readahead: Store folio order in struct
file_ra_state
On Mon 09-06-25 10:27:26, Ryan Roberts wrote:
> Previously the folio order of the previous readahead request was
> inferred from the folio who's readahead marker was hit. But due to the
> way we have to round to non-natural boundaries sometimes, this first
> folio in the readahead block is often smaller than the preferred order
> for that request. This means that for cases where the initial sync
> readahead is poorly aligned, the folio order will ramp up much more
> slowly.
>
> So instead, let's store the order in struct file_ra_state so we are not
> affected by any required alignment. We previously made enough room in
> the struct for a 16 order field. This should be plenty big enough since
> we are limited to MAX_PAGECACHE_ORDER anyway, which is certainly never
> larger than ~20.
>
> Since we now pass order in struct file_ra_state, page_cache_ra_order()
> no longer needs it's new_order parameter, so let's remove that.
>
> Worked example:
>
> Here we are touching pages 17-256 sequentially just as we did in the
> previous commit, but now that we are remembering the preferred order
> explicitly, we no longer have the slow ramp up problem. Note
> specifically that we no longer have 2 rounds (2x ~128K) of order-2
> folios:
>
> TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER RA
> ----- ---------- ---------- ---------- ------- ------- ----- ----- --
> HOLE 0x00000000 0x00001000 4096 0 1 1
> FOLIO 0x00001000 0x00002000 4096 1 2 1 0
> FOLIO 0x00002000 0x00003000 4096 2 3 1 0
> FOLIO 0x00003000 0x00004000 4096 3 4 1 0
> FOLIO 0x00004000 0x00005000 4096 4 5 1 0
> FOLIO 0x00005000 0x00006000 4096 5 6 1 0
> FOLIO 0x00006000 0x00007000 4096 6 7 1 0
> FOLIO 0x00007000 0x00008000 4096 7 8 1 0
> FOLIO 0x00008000 0x00009000 4096 8 9 1 0
> FOLIO 0x00009000 0x0000a000 4096 9 10 1 0
> FOLIO 0x0000a000 0x0000b000 4096 10 11 1 0
> FOLIO 0x0000b000 0x0000c000 4096 11 12 1 0
> FOLIO 0x0000c000 0x0000d000 4096 12 13 1 0
> FOLIO 0x0000d000 0x0000e000 4096 13 14 1 0
> FOLIO 0x0000e000 0x0000f000 4096 14 15 1 0
> FOLIO 0x0000f000 0x00010000 4096 15 16 1 0
> FOLIO 0x00010000 0x00011000 4096 16 17 1 0
> FOLIO 0x00011000 0x00012000 4096 17 18 1 0
> FOLIO 0x00012000 0x00013000 4096 18 19 1 0
> FOLIO 0x00013000 0x00014000 4096 19 20 1 0
> FOLIO 0x00014000 0x00015000 4096 20 21 1 0
> FOLIO 0x00015000 0x00016000 4096 21 22 1 0
> FOLIO 0x00016000 0x00017000 4096 22 23 1 0
> FOLIO 0x00017000 0x00018000 4096 23 24 1 0
> FOLIO 0x00018000 0x00019000 4096 24 25 1 0
> FOLIO 0x00019000 0x0001a000 4096 25 26 1 0
> FOLIO 0x0001a000 0x0001b000 4096 26 27 1 0
> FOLIO 0x0001b000 0x0001c000 4096 27 28 1 0
> FOLIO 0x0001c000 0x0001d000 4096 28 29 1 0
> FOLIO 0x0001d000 0x0001e000 4096 29 30 1 0
> FOLIO 0x0001e000 0x0001f000 4096 30 31 1 0
> FOLIO 0x0001f000 0x00020000 4096 31 32 1 0
> FOLIO 0x00020000 0x00021000 4096 32 33 1 0
> FOLIO 0x00021000 0x00022000 4096 33 34 1 0
> FOLIO 0x00022000 0x00024000 8192 34 36 2 1
> FOLIO 0x00024000 0x00028000 16384 36 40 4 2
> FOLIO 0x00028000 0x0002c000 16384 40 44 4 2
> FOLIO 0x0002c000 0x00030000 16384 44 48 4 2
> FOLIO 0x00030000 0x00034000 16384 48 52 4 2
> FOLIO 0x00034000 0x00038000 16384 52 56 4 2
> FOLIO 0x00038000 0x0003c000 16384 56 60 4 2
> FOLIO 0x0003c000 0x00040000 16384 60 64 4 2
> FOLIO 0x00040000 0x00050000 65536 64 80 16 4
> FOLIO 0x00050000 0x00060000 65536 80 96 16 4
> FOLIO 0x00060000 0x00080000 131072 96 128 32 5
> FOLIO 0x00080000 0x000a0000 131072 128 160 32 5
> FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5
> FOLIO 0x000c0000 0x000e0000 131072 192 224 32 5
> FOLIO 0x000e0000 0x00100000 131072 224 256 32 5
> FOLIO 0x00100000 0x00120000 131072 256 288 32 5
> FOLIO 0x00120000 0x00140000 131072 288 320 32 5 Y
> HOLE 0x00140000 0x00800000 7077888 320 2048 1728
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
Looks good! Feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> include/linux/fs.h | 2 ++
> mm/filemap.c | 6 ++++--
> mm/internal.h | 3 +--
> mm/readahead.c | 21 +++++++++++++--------
> 4 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 87e7d5790e43..b5172b691f97 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1041,6 +1041,7 @@ struct fown_struct {
> * and so were/are genuinely "ahead". Start next readahead when
> * the first of these pages is accessed.
> * @ra_pages: Maximum size of a readahead request, copied from the bdi.
> + * @order: Preferred folio order used for most recent readahead.
> * @mmap_miss: How many mmap accesses missed in the page cache.
> * @prev_pos: The last byte in the most recent read request.
> *
> @@ -1052,6 +1053,7 @@ struct file_ra_state {
> unsigned int size;
> unsigned int async_size;
> unsigned int ra_pages;
> + unsigned short order;
> unsigned short mmap_miss;
> loff_t prev_pos;
> };
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 7bb4ffca8487..4b5c8d69f04c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3232,7 +3232,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> if (!(vm_flags & VM_RAND_READ))
> ra->size *= 2;
> ra->async_size = HPAGE_PMD_NR;
> - page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
> + ra->order = HPAGE_PMD_ORDER;
> + page_cache_ra_order(&ractl, ra);
> return fpin;
> }
> #endif
> @@ -3268,8 +3269,9 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> ra->size = ra->ra_pages;
> ra->async_size = ra->ra_pages / 4;
> + ra->order = 0;
> ractl._index = ra->start;
> - page_cache_ra_order(&ractl, ra, 0);
> + page_cache_ra_order(&ractl, ra);
> return fpin;
> }
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 6b8ed2017743..f91688e2894f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -436,8 +436,7 @@ void zap_page_range_single_batched(struct mmu_gather *tlb,
> int folio_unmap_invalidate(struct address_space *mapping, struct folio *folio,
> gfp_t gfp);
>
> -void page_cache_ra_order(struct readahead_control *, struct file_ra_state *,
> - unsigned int order);
> +void page_cache_ra_order(struct readahead_control *, struct file_ra_state *);
> void force_page_cache_ra(struct readahead_control *, unsigned long nr);
> static inline void force_page_cache_readahead(struct address_space *mapping,
> struct file *file, pgoff_t index, unsigned long nr_to_read)
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 87be20ae00d0..95a24f12d1e7 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -457,7 +457,7 @@ static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
> }
>
> void page_cache_ra_order(struct readahead_control *ractl,
> - struct file_ra_state *ra, unsigned int new_order)
> + struct file_ra_state *ra)
> {
> struct address_space *mapping = ractl->mapping;
> pgoff_t start = readahead_index(ractl);
> @@ -468,9 +468,12 @@ void page_cache_ra_order(struct readahead_control *ractl,
> unsigned int nofs;
> int err = 0;
> gfp_t gfp = readahead_gfp_mask(mapping);
> + unsigned int new_order = ra->order;
>
> - if (!mapping_large_folio_support(mapping))
> + if (!mapping_large_folio_support(mapping)) {
> + ra->order = 0;
> goto fallback;
> + }
>
> limit = min(limit, index + ra->size - 1);
>
> @@ -478,6 +481,8 @@ void page_cache_ra_order(struct readahead_control *ractl,
> new_order = min_t(unsigned int, new_order, ilog2(ra->size));
> new_order = max(new_order, min_order);
>
> + ra->order = new_order;
> +
> /* See comment in page_cache_ra_unbounded() */
> nofs = memalloc_nofs_save();
> filemap_invalidate_lock_shared(mapping);
> @@ -609,8 +614,9 @@ void page_cache_sync_ra(struct readahead_control *ractl,
> ra->size = min(contig_count + req_count, max_pages);
> ra->async_size = 1;
> readit:
> + ra->order = 0;
> ractl->_index = ra->start;
> - page_cache_ra_order(ractl, ra, 0);
> + page_cache_ra_order(ractl, ra);
> }
> EXPORT_SYMBOL_GPL(page_cache_sync_ra);
>
> @@ -621,7 +627,6 @@ void page_cache_async_ra(struct readahead_control *ractl,
> struct file_ra_state *ra = ractl->ra;
> pgoff_t index = readahead_index(ractl);
> pgoff_t expected, start, end, aligned_end, align;
> - unsigned int order = folio_order(folio);
>
> /* no readahead */
> if (!ra->ra_pages)
> @@ -644,7 +649,7 @@ void page_cache_async_ra(struct readahead_control *ractl,
> * Ramp up sizes, and push forward the readahead window.
> */
> expected = round_down(ra->start + ra->size - ra->async_size,
> - 1UL << order);
> + 1UL << folio_order(folio));
> if (index == expected) {
> ra->start += ra->size;
> /*
> @@ -673,15 +678,15 @@ void page_cache_async_ra(struct readahead_control *ractl,
> ra->size += req_count;
> ra->size = get_next_ra_size(ra, max_pages);
> readit:
> - order += 2;
> - align = 1UL << min(order, ffs(max_pages) - 1);
> + ra->order += 2;
> + align = 1UL << min(ra->order, ffs(max_pages) - 1);
> end = ra->start + ra->size;
> aligned_end = round_down(end, align);
> if (aligned_end > ra->start)
> ra->size -= end - aligned_end;
> ra->async_size = ra->size;
> ractl->_index = ra->start;
> - page_cache_ra_order(ractl, ra, order);
> + page_cache_ra_order(ractl, ra);
> }
> EXPORT_SYMBOL_GPL(page_cache_async_ra);
>
> --
> 2.43.0
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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