On 2025/6/17 21:13, Sheng Yong wrote:
On 6/17/25 19:37, Chao Yu via Linux-f2fs-devel wrote:
On 6/17/25 13:55, Jianan Huang wrote:
When fewer pages are read, nr_pages may be smaller than nr_cpages. Due
to the nr_vecs limit, the compressed pages will be split into multiple
bios and then merged at the block level. In this case, nr_cpages should
be used to pre-allocate bvecs.

Signed-off-by: Jianan Huang <huangjia...@xiaomi.com>
Signed-off-by: Sheng Yong <shengyo...@xiaomi.com>
---
  fs/f2fs/data.c | 10 +++++-----
  1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 31e892842625..c7773b09d83f 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2303,7 +2303,8 @@ int f2fs_read_multi_pages(struct compress_ctx *cc, struct bio **bio_ret,
          }
          if (!bio) {
-            bio = f2fs_grab_read_bio(inode, blkaddr, nr_pages,
+            bio = f2fs_grab_read_bio(inode, blkaddr,
+                    max(nr_pages, cc->nr_cpages) - i,

Hi Jianan,

e.g.

User wants to read page [1, 5],
page #1,2,3,4 locates in compressed block #1000,1001,1003,
page #5 locate in compressed block #1004,1005

It submits first bio w/ block #1000,1001
It allocates second bio w/ size of max(nr_pages=1, nr_cpages=3) - 2 = 1 ?
However block #1003 and block #1004,1005 can be readed in one bio, we
should allocate larger bio for last continuous blocks which cross clusters.

Hi, Chao,

I think `max(nr_pages, cc->nr_cpages) - i` can reserve enough vectors in bio
for later reads. IIUC, the case above is:

read page #1,2,3,4 at blkaddr #1000,1001,1003:
  * nr_pages=5, cpages=3, for the first bio1, vec=max(5,3)-0=5 (2 vecs are used)                           for the second bio2, vec=max(5,3)-2=3 (1 vec is used)

Hi Yong,

Sorry for the delay.

About the second bio2 (vec=max(5,3)-2=3), in this patch, we pass nr_pages instead of max_nr_pages, nr_pages will be decreased to 1 rather than 5 when we allocate new bio. Because we add page #1,2,3,4 to decompress_ctx first, and then read cluster entirely, at that time, nr_pages is 1? IIRC.

Thanks,

read page #5 at blkaddr #1004,1005, prev bio2 is still available
   * nr_pages=1, cpages=2, prev bio2, 2 vecs left


For case: page #1,2,3,4 at compressed blkaddr #1000,1001,1003
           page #5,6,7,8 at compressed blkaddr #1004,1005,1006
If we are reading page[1,5], we could do calculation as the following?

     max_nr_pages=align(nr_pages, cluster_size)
     max(max_nr_pages, cc->nr_cpages) - i


thanks,
shengyong

                      f2fs_ra_op_flags(rac),
                      folio->index, for_write);
              if (IS_ERR(bio)) {
@@ -2373,7 +2374,6 @@ static int f2fs_mpage_readpages(struct inode *inode,
      pgoff_t index;
  #endif
      unsigned nr_pages = rac ? readahead_count(rac) : 1;
-    unsigned max_nr_pages = nr_pages;

Maybe we can align both start and end of read range w/ cluster_size, and use
start and end for max_nr_pages calculation, then pass it to
f2fs_read_{multi,single}_pages(), something like this?

max_nr_pages = round_up(end_idx, cluster_size) -
        round_down(start_idx, cluster_size);

Its size should always cover size of all cpage and/or rpage.

Thanks,

      int ret = 0;
      map.m_pblk = 0;
@@ -2400,7 +2400,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
          /* there are remained compressed pages, submit them */
          if (!f2fs_cluster_can_merge_page(&cc, index)) {
              ret = f2fs_read_multi_pages(&cc, &bio,
-                        max_nr_pages,
+                        nr_pages,
                          &last_block_in_bio,
                          rac, false);
              f2fs_destroy_compress_ctx(&cc, false);
@@ -2432,7 +2432,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
  read_single_page:
  #endif
-        ret = f2fs_read_single_page(inode, folio, max_nr_pages, &map,
+        ret = f2fs_read_single_page(inode, folio, nr_pages, &map,
                      &bio, &last_block_in_bio, rac);
          if (ret) {
  #ifdef CONFIG_F2FS_FS_COMPRESSION
@@ -2450,7 +2450,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
              /* last page */
              if (nr_pages == 1 && !f2fs_cluster_is_empty(&cc)) {
                  ret = f2fs_read_multi_pages(&cc, &bio,
-                            max_nr_pages,
+                            nr_pages,
                              &last_block_in_bio,
                              rac, false);
                  f2fs_destroy_compress_ctx(&cc, false);



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