The quilt patch titled
Subject: mm/filemap: add read support for RWF_DONTCACHE
has been removed from the -mm tree. Its filename was
mm-filemap-add-read-support-for-rwf_dontcache.patch
This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: Jens Axboe <axboe@xxxxxxxxx>
Subject: mm/filemap: add read support for RWF_DONTCACHE
Date: Fri, 20 Dec 2024 08:47:46 -0700
Add RWF_DONTCACHE as a read operation flag, which means that any data read
wil be removed from the page cache upon completion. Uses the page cache
to synchronize, and simply prunes folios that were instantiated when the
operation completes. While it would be possible to use private pages for
this, using the page cache as synchronization is handy for a variety of
reasons:
1) No special truncate magic is needed
2) Async buffered reads need some place to serialize, using the page
cache is a lot easier than writing extra code for this
3) The pruning cost is pretty reasonable
and the code to support this is much simpler as a result.
You can think of uncached buffered IO as being the much more attractive
cousin of O_DIRECT - it has none of the restrictions of O_DIRECT. Yes, it
will copy the data, but unlike regular buffered IO, it doesn't run into
the unpredictability of the page cache in terms of reclaim. As an
example, on a test box with 32 drives, reading them with buffered IO looks
as follows:
Reading bs 65536, uncached 0
1s: 145945MB/sec
2s: 158067MB/sec
3s: 157007MB/sec
4s: 148622MB/sec
5s: 118824MB/sec
6s: 70494MB/sec
7s: 41754MB/sec
8s: 90811MB/sec
9s: 92204MB/sec
10s: 95178MB/sec
11s: 95488MB/sec
12s: 95552MB/sec
13s: 96275MB/sec
where it's quite easy to see where the page cache filled up, and
performance went from good to erratic, and finally settles at a much
lower rate. Looking at top while this is ongoing, we see:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7535 root 20 0 267004 0 0 S 3199 0.0 8:40.65 uncached
3326 root 20 0 0 0 0 R 100.0 0.0 0:16.40 kswapd4
3327 root 20 0 0 0 0 R 100.0 0.0 0:17.22 kswapd5
3328 root 20 0 0 0 0 R 100.0 0.0 0:13.29 kswapd6
3332 root 20 0 0 0 0 R 100.0 0.0 0:11.11 kswapd10
3339 root 20 0 0 0 0 R 100.0 0.0 0:16.25 kswapd17
3348 root 20 0 0 0 0 R 100.0 0.0 0:16.40 kswapd26
3343 root 20 0 0 0 0 R 100.0 0.0 0:16.30 kswapd21
3344 root 20 0 0 0 0 R 100.0 0.0 0:11.92 kswapd22
3349 root 20 0 0 0 0 R 100.0 0.0 0:16.28 kswapd27
3352 root 20 0 0 0 0 R 99.7 0.0 0:11.89 kswapd30
3353 root 20 0 0 0 0 R 96.7 0.0 0:16.04 kswapd31
3329 root 20 0 0 0 0 R 96.4 0.0 0:11.41 kswapd7
3345 root 20 0 0 0 0 R 96.4 0.0 0:13.40 kswapd23
3330 root 20 0 0 0 0 S 91.1 0.0 0:08.28 kswapd8
3350 root 20 0 0 0 0 S 86.8 0.0 0:11.13 kswapd28
3325 root 20 0 0 0 0 S 76.3 0.0 0:07.43 kswapd3
3341 root 20 0 0 0 0 S 74.7 0.0 0:08.85 kswapd19
3334 root 20 0 0 0 0 S 71.7 0.0 0:10.04 kswapd12
3351 root 20 0 0 0 0 R 60.5 0.0 0:09.59 kswapd29
3323 root 20 0 0 0 0 R 57.6 0.0 0:11.50 kswapd1
[...]
which is just showing a partial list of the 32 kswapd threads that are
running mostly full tilt, burning ~28 full CPU cores.
If the same test case is run with RWF_DONTCACHE set for the buffered read,
the output looks as follows:
Reading bs 65536, uncached 0
1s: 153144MB/sec
2s: 156760MB/sec
3s: 158110MB/sec
4s: 158009MB/sec
5s: 158043MB/sec
6s: 157638MB/sec
7s: 157999MB/sec
8s: 158024MB/sec
9s: 157764MB/sec
10s: 157477MB/sec
11s: 157417MB/sec
12s: 157455MB/sec
13s: 157233MB/sec
14s: 156692MB/sec
which is just chugging along at ~155GB/sec of read performance. Looking
at top, we see:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7961 root 20 0 267004 0 0 S 3180 0.0 5:37.95 uncached
8024 axboe 20 0 14292 4096 0 R 1.0 0.0 0:00.13 top
where just the test app is using CPU, no reclaim is taking place outside
of the main thread. Not only is performance 65% better, it's also using
half the CPU to do it.
Link: https://lkml.kernel.org/r/20241220154831.1086649-9-axboe@xxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Cc: Brian Foster <bfoster@xxxxxxxxxx>
Cc: Chris Mason <clm@xxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 28 ++++++++++++++++++++++++++--
mm/swap.c | 2 ++
2 files changed, 28 insertions(+), 2 deletions(-)
--- a/mm/filemap.c~mm-filemap-add-read-support-for-rwf_dontcache
+++ a/mm/filemap.c
@@ -2445,6 +2445,8 @@ static int filemap_create_folio(struct k
folio = filemap_alloc_folio(mapping_gfp_mask(mapping), min_order);
if (!folio)
return -ENOMEM;
+ if (iocb->ki_flags & IOCB_DONTCACHE)
+ __folio_set_dropbehind(folio);
/*
* Protect against truncate / hole punch. Grabbing invalidate_lock
@@ -2490,6 +2492,8 @@ static int filemap_readahead(struct kioc
if (iocb->ki_flags & IOCB_NOIO)
return -EAGAIN;
+ if (iocb->ki_flags & IOCB_DONTCACHE)
+ ractl.dropbehind = 1;
page_cache_async_ra(&ractl, folio, last_index - folio->index);
return 0;
}
@@ -2519,6 +2523,8 @@ retry:
return -EAGAIN;
if (iocb->ki_flags & IOCB_NOWAIT)
flags = memalloc_noio_save();
+ if (iocb->ki_flags & IOCB_DONTCACHE)
+ ractl.dropbehind = 1;
page_cache_sync_ra(&ractl, last_index - index);
if (iocb->ki_flags & IOCB_NOWAIT)
memalloc_noio_restore(flags);
@@ -2566,6 +2572,20 @@ static inline bool pos_same_folio(loff_t
return (pos1 >> shift == pos2 >> shift);
}
+static void filemap_end_dropbehind_read(struct address_space *mapping,
+ struct folio *folio)
+{
+ if (!folio_test_dropbehind(folio))
+ return;
+ if (folio_test_writeback(folio) || folio_test_dirty(folio))
+ return;
+ if (folio_trylock(folio)) {
+ if (folio_test_clear_dropbehind(folio))
+ folio_unmap_invalidate(mapping, folio, 0);
+ folio_unlock(folio);
+ }
+}
+
/**
* filemap_read - Read data from the page cache.
* @iocb: The iocb to read.
@@ -2679,8 +2699,12 @@ ssize_t filemap_read(struct kiocb *iocb,
}
}
put_folios:
- for (i = 0; i < folio_batch_count(&fbatch); i++)
- folio_put(fbatch.folios[i]);
+ for (i = 0; i < folio_batch_count(&fbatch); i++) {
+ struct folio *folio = fbatch.folios[i];
+
+ filemap_end_dropbehind_read(mapping, folio);
+ folio_put(folio);
+ }
folio_batch_init(&fbatch);
} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);
--- a/mm/swap.c~mm-filemap-add-read-support-for-rwf_dontcache
+++ a/mm/swap.c
@@ -448,6 +448,8 @@ static bool lru_gen_clear_refs(struct fo
*/
void folio_mark_accessed(struct folio *folio)
{
+ if (folio_test_dropbehind(folio))
+ return;
if (lru_gen_enabled()) {
lru_gen_inc_refs(folio);
return;
_
Patches currently in -mm which might be from axboe@xxxxxxxxx are
