[PATCH 08/17] mm/filemap: add read support for RWF_UNCACHED

Jens Axboe
2024-11-14T15:28:13Z

Add RWF_UNCACHED 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_UNCACHED 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.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
mm/filemap.c | 28 ++++++++++++++++++++++++++--
mm/swap.c | 2 ++
2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 02d9cb585195..3d0614ea5f59 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2474,6 +2474,8 @@ static int filemap_create_folio(struct kiocb *iocb,
folio = filemap_alloc_folio(mapping_gfp_mask(mapping), min_order);
if (!folio)
return -ENOMEM;
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ __folio_set_uncached(folio);

/*
* Protect against truncate / hole punch. Grabbing invalidate_lock
@@ -2519,6 +2521,8 @@ static int filemap_readahead(struct kiocb *iocb, struct file *file,

if (iocb->ki_flags & IOCB_NOIO)
return -EAGAIN;
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ ractl.uncached = 1;
page_cache_async_ra(&ractl, folio, last_index - folio->index);
return 0;
}
@@ -2548,6 +2552,8 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
return -EAGAIN;
if (iocb->ki_flags & IOCB_NOWAIT)
flags = memalloc_noio_save();
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ ractl.uncached = 1;
page_cache_sync_ra(&ractl, last_index - index);
if (iocb->ki_flags & IOCB_NOWAIT)
memalloc_noio_restore(flags);
@@ -2595,6 +2601,20 @@ static inline bool pos_same_folio(loff_t pos1, loff_t pos2, struct folio *folio)
return (pos1 >> shift == pos2 >> shift);
}

+static void filemap_uncached_read(struct address_space *mapping,
+ struct folio *folio)
+{
+ if (!folio_test_uncached(folio))
+ return;
+ if (folio_test_writeback(folio))
+ return;
+ if (folio_test_clear_uncached(folio)) {
+ folio_lock(folio);
+ folio_unmap_invalidate(mapping, folio, 0);
+ folio_unlock(folio);
+ }
+}
+
/**
* filemap_read - Read data from the page cache.
* @iocb: The iocb to read.
@@ -2706,8 +2726,12 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
}
}
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_uncached_read(mapping, folio);
+ folio_put(folio);
+ }
folio_batch_init(&fbatch);
} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);

diff --git a/mm/swap.c b/mm/swap.c
index b8e3259ea2c4..542f298d3dcd 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -472,6 +472,8 @@ static void folio_inc_refs(struct folio *folio)
*/
void folio_mark_accessed(struct folio *folio)
{
+ if (folio_test_uncached(folio))
+ return;
if (lru_gen_enabled()) {
folio_inc_refs(folio);
return;
--
2.45.2