Re: [External Mail]Re: [PATCH v3] f2fs: avoid splitting bio when reading multiple pages

[Huang Jianan <huangjianan@xiaomi.com>]

On 2025/6/25 16:45, Chao Yu wrote:
> 
> On 6/25/25 14:49, 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.
>> To handle this case, align max_nr_pages to cluster_size, which should be
>> enough for all compressed pages.
>>
>> Signed-off-by: Jianan Huang <huangjianan@xiaomi.com>
>> Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
>> ---
>> Changes since v2:
>> - Initialize index only for compressed files.
>> Changes since v1:
>> - Use aligned nr_pages instead of nr_cpages to pre-allocate bvecs.
>>
>>   fs/f2fs/data.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 31e892842625..d071d9f6a811 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -2303,7 +2303,7 @@ 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, nr_pages - i,
> 
> Jianan,
> 
> Another case:
> 
> read page #0,1,2,3 from block #1000,1001,1002, cluster_size=4.
> 
> nr_pages=4
> max_nr_pages=round_up(0+4,4)-round_down(0,4)=4
> 
> f2fs_mpage_readpages() calls f2fs_read_multi_pages() when nr_pages=1, at
> that time, max_nr_pages equals to 1 as well.
> 
> f2fs_grab_read_bio(..., 1 - 0,...) allocate bio w/ 1 vec capacity, however,
> we need at least 3 vecs to merge all cpages, right?
> 

Hi, chao,

If we don't align nr_pages, then when entering f2fs_read_multi_pages,
we have nr_pages pages left, which belong to other clusters.
If this is the last page, we can simply pass nr_pages = 0.

When allocating bio, we need:
1. The cpages remaining in the current cluster, which should be 
(nr_capges - i).
2. The maximum cpages remaining in other clusters, which should be 
max(nr_pages, cc->nr_cpages).

So (nr_capges - i) + max(nr_pages, nr_cpages), should be enough for all 
vecs?

Thanks,


> Thanks,
> 
>>                                        f2fs_ra_op_flags(rac),
>>                                        folio->index, for_write);
>>                        if (IS_ERR(bio)) {
>> @@ -2376,6 +2376,14 @@ static int f2fs_mpage_readpages(struct inode *inode,
>>        unsigned max_nr_pages = nr_pages;
>>        int ret = 0;
>>
>> +#ifdef CONFIG_F2FS_FS_COMPRESSION
>> +     if (f2fs_compressed_file(inode)) {
>> +             index = rac ? readahead_index(rac) : folio->index;
>> +             max_nr_pages = round_up(index + nr_pages, cc.cluster_size) -
>> +                             round_down(index, cc.cluster_size);
>> +     }
>> +#endif
>> +
>>        map.m_pblk = 0;
>>        map.m_lblk = 0;
>>        map.m_len = 0;
>> @@ -2385,7 +2393,7 @@ static int f2fs_mpage_readpages(struct inode *inode,
>>        map.m_seg_type = NO_CHECK_TYPE;
>>        map.m_may_create = false;
>>
>> -     for (; nr_pages; nr_pages--) {
>> +     for (; nr_pages; nr_pages--, max_nr_pages--) {
>>                if (rac) {
>>                        folio = readahead_folio(rac);
>>                        prefetchw(&folio->flags);
>