Re: [f2fs-dev] [PATCH] f2fs: f2fs supports uncached buffered I/O read

[hanqi via Linux-f2fs-devel]

在 2025/7/28 15:38, Chao Yu 写道:

On 7/25/25 15:53, Qi Han wrote:

Jens has already completed the development of uncached buffered I/O
in git [1], and in f2fs, uncached buffered I/O read can be enabled
simply by setting the FOP_DONTCACHE flag in f2fs_file_operations.

IIUC, we may suffer lock issue when we call pwritev(.. ,RWF_DONTCACHE)?
as Jen mentioned in below path, right?

soft-irq
- folio_end_writeback()
  - filemap_end_dropbehind_write()
   - filemap_end_dropbehind()
    - folio_unmap_invalidate()
     - lock i_lock

Thanks,

That's how I understand it.

I have been testing a use case locally, which aligns with Jens' test
case [2]. In the read scenario, using uncached buffer I/O results in
more stable read performance and a lower load on the background memory
reclaim thread (kswapd). So let's enable uncached buffer I/O reads on
F2FS.

Read test data without using uncached buffer I/O:
reading bs 32768, uncached 0
    1s: 1856MB/sec, MB=1856
    2s: 1907MB/sec, MB=3763
    3s: 1830MB/sec, MB=5594
    4s: 1745MB/sec, MB=7333
    5s: 1829MB/sec, MB=9162
    6s: 1903MB/sec, MB=11075
    7s: 1878MB/sec, MB=12942
    8s: 1763MB/sec, MB=14718
    9s: 1845MB/sec, MB=16549
   10s: 1915MB/sec, MB=18481
   11s: 1831MB/sec, MB=20295
   12s: 1750MB/sec, MB=22066
   13s: 1787MB/sec, MB=23832
   14s: 1913MB/sec, MB=25769
   15s: 1898MB/sec, MB=27668
   16s: 1795MB/sec, MB=29436
   17s: 1812MB/sec, MB=31248
   18s: 1890MB/sec, MB=33139
   19s: 1880MB/sec, MB=35020
   20s: 1754MB/sec, MB=36810

08:36:26      UID       PID    %usr %system  %guest   %wait    %CPU   CPU  
Command
08:36:27        0        93    0.00    0.00    0.00    0.00    0.00     7  
kswapd0
08:36:28        0        93    0.00    0.00    0.00    0.00    0.00     7  
kswapd0
08:36:29        0        93    0.00    0.00    0.00    0.00    0.00     7  
kswapd0
08:36:30        0        93    0.00   56.00    0.00    0.00   56.00     7  
kswapd0
08:36:31        0        93    0.00   73.00    0.00    0.00   73.00     7  
kswapd0
08:36:32        0        93    0.00   83.00    0.00    0.00   83.00     7  
kswapd0
08:36:33        0        93    0.00   75.00    0.00    0.00   75.00     7  
kswapd0
08:36:34        0        93    0.00   81.00    0.00    0.00   81.00     7  
kswapd0
08:36:35        0        93    0.00   54.00    0.00    1.00   54.00     2  
kswapd0
08:36:36        0        93    0.00   61.00    0.00    0.00   61.00     0  
kswapd0
08:36:37        0        93    0.00   68.00    0.00    0.00   68.00     7  
kswapd0
08:36:38        0        93    0.00   53.00    0.00    0.00   53.00     2  
kswapd0
08:36:39        0        93    0.00   82.00    0.00    0.00   82.00     7  
kswapd0
08:36:40        0        93    0.00   77.00    0.00    0.00   77.00     1  
kswapd0
08:36:41        0        93    0.00   74.00    0.00    1.00   74.00     7  
kswapd0
08:36:42        0        93    0.00   71.00    0.00    0.00   71.00     7  
kswapd0
08:36:43        0        93    0.00   78.00    0.00    0.00   78.00     7  
kswapd0
08:36:44        0        93    0.00   85.00    0.00    0.00   85.00     7  
kswapd0
08:36:45        0        93    0.00   83.00    0.00    0.00   83.00     7  
kswapd0
08:36:46        0        93    0.00   70.00    0.00    0.00   70.00     7  
kswapd0
08:36:47        0        93    0.00   78.00    0.00    1.00   78.00     2  
kswapd0
08:36:48        0        93    0.00   81.00    0.00    0.00   81.00     3  
kswapd0
08:36:49        0        93    0.00   54.00    0.00    0.00   54.00     7  
kswapd0
08:36:50        0        93    0.00   76.00    0.00    0.00   76.00     1  
kswapd0
08:36:51        0        93    0.00   75.00    0.00    0.00   75.00     0  
kswapd0
08:36:52        0        93    0.00   73.00    0.00    0.00   73.00     7  
kswapd0
08:36:53        0        93    0.00   61.00    0.00    1.00   61.00     7  
kswapd0
08:36:54        0        93    0.00   80.00    0.00    0.00   80.00     7  
kswapd0
08:36:55        0        93    0.00   64.00    0.00    0.00   64.00     7  
kswapd0
08:36:56        0        93    0.00   56.00    0.00    0.00   56.00     7  
kswapd0
08:36:57        0        93    0.00   26.00    0.00    0.00   26.00     2  
kswapd0
08:36:58        0        93    0.00   24.00    0.00    1.00   24.00     3  
kswapd0
08:36:59        0        93    0.00   22.00    0.00    1.00   22.00     3  
kswapd0
08:37:00        0        93    0.00   15.84    0.00    0.00   15.84     3  
kswapd0
08:37:01        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:37:02        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0

Read test data after using uncached buffer I/O:
reading bs 32768, uncached 1
    1s: 1863MB/sec, MB=1863
    2s: 1903MB/sec, MB=3766
    3s: 1860MB/sec, MB=5627
    4s: 1864MB/sec, MB=7491
    5s: 1860MB/sec, MB=9352
    6s: 1854MB/sec, MB=11206
    7s: 1874MB/sec, MB=13081
    8s: 1874MB/sec, MB=14943
    9s: 1840MB/sec, MB=16798
   10s: 1849MB/sec, MB=18647
   11s: 1863MB/sec, MB=20511
   12s: 1798MB/sec, MB=22310
   13s: 1897MB/sec, MB=24207
   14s: 1817MB/sec, MB=26025
   15s: 1893MB/sec, MB=27918
   16s: 1917MB/sec, MB=29836
   17s: 1863MB/sec, MB=31699
   18s: 1904MB/sec, MB=33604
   19s: 1894MB/sec, MB=35499
   20s: 1907MB/sec, MB=37407

08:38:00      UID       PID    %usr %system  %guest   %wait    %CPU   CPU  
Command
08:38:01        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:02        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:03        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:04        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:05        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:06        0        93    0.00    1.00    0.00    1.00    1.00     0  
kswapd0
08:38:07        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:08        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:09        0        93    0.00    1.00    0.00    0.00    1.00     1  
kswapd0
08:38:10        0        93    0.00    0.00    0.00    0.00    0.00     1  
kswapd0
08:38:11        0        93    0.00    0.00    0.00    0.00    0.00     1  
kswapd0
08:38:12        0        93    0.00    0.00    0.00    0.00    0.00     1  
kswapd0
08:38:13        0        93    0.00    0.00    0.00    0.00    0.00     1  
kswapd0
08:38:14        0        93    0.00    0.00    0.00    0.00    0.00     1  
kswapd0
08:38:15        0        93    0.00    3.00    0.00    0.00    3.00     0  
kswapd0
08:38:16        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:17        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:18        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:19        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:20        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:21        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:22        0        93    0.00    0.00    0.00    0.00    0.00     0  
kswapd0
08:38:23        0        93    0.00    3.00    0.00    0.00    3.00     4  
kswapd0
08:38:24        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:25        0        93    0.00    0.00    0.00    0.00    0.00     4  
kswapd0
08:38:26        0        93    0.00    4.00    0.00    0.00    4.00     3  
kswapd0
08:38:27        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:28        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:29        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:30        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:31        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:32        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0
08:38:33        0        93    0.00    0.00    0.00    0.00    0.00     3  
kswapd0

[1]
https://lore.kernel.org/all/20241220154831.1086649-10-ax...@kernel.dk/T/#m58520a94b46f543d82db3711453dfc7bb594b2b0

[2]
https://pastebin.com/u8eCBzB5

Signed-off-by: Qi Han <ha...@vivo.com>
---
  fs/f2fs/file.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 696131e655ed..d8da1fc2febf 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -5425,5 +5425,5 @@ const struct file_operations f2fs_file_operations = {
        .splice_read    = f2fs_file_splice_read,
        .splice_write   = iter_file_splice_write,
        .fadvise        = f2fs_file_fadvise,
-       .fop_flags      = FOP_BUFFER_RASYNC,
+       .fop_flags      = FOP_BUFFER_RASYNC | FOP_DONTCACHE,
  };

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