Re: [PATCH v3 3/4] btrfs: submit read time repair only for each corrupted sector

[David Sterba <dsterba@suse.cz>]

On Mon, May 03, 2021 at 10:08:55AM +0800, Qu Wenruo wrote:
> Currently btrfs_submit_read_repair() has some extra check on whether the
> failed bio needs extra validation for repair.
> 
> But we can avoid all these extra mechanism if we submit the repair for
> each sector.
> 
> By this, each read repair can be easily handled without the need to
> verify which sector is corrupted.
> 
> This will also benefit subpage, as one subpage bvec can contain several
> sectors, making the extra verification more complex.
> 
> So this patch will:
> - Introduce repair_one_sector()
>   The main code submitting repair, which is more or less the same as old
>   btrfs_submit_read_repair().
>   But this time, it only repair one sector.
> 
> - Make btrfs_submit_read_repair() to handle sectors differently
>   For sectors without csum error, just release them like what we did
>   in end_bio_extent_readpage().
>   Although in this context we don't have process_extent structure, thus
>   we have to do extent tree operations sector by sector.
>   This is slower, but since it's only in csum mismatch path, it should
>   be fine.
> 
>   For sectors with csum error, we submit repair for each sector.
> 
> This patch will focus on the change on the repair path, the extra
> validation code is still kept as is, and will be cleaned up later.

This leaves btrfs_io_needs_validation unused and compiler warns about
that but it gets removed in the next patch so that's ok.

I did some minor style fixups

--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2706,7 +2706,7 @@ static void end_page_read(struct page *page, bool uptodate, u64 start, u32 len)
        struct btrfs_fs_info *fs_info = btrfs_sb(page->mapping->host->i_sb);
 
        ASSERT(page_offset(page) <= start &&
-               start + len <= page_offset(page) + PAGE_SIZE);
+              start + len <= page_offset(page) + PAGE_SIZE);
 
        if (uptodate) {
                btrfs_page_set_uptodate(fs_info, page, start, len);
@@ -2734,7 +2734,7 @@ blk_status_t btrfs_submit_read_repair(struct inode *inode,
 {
        struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
        const u32 sectorsize = fs_info->sectorsize;
-       int nr_bits = (end + 1 - start) / sectorsize;
+       const int nr_bits = (end + 1 - start) / fs_info->sectorsize_bits;
        int i;
 
        BUG_ON(bio_op(failed_bio) == REQ_OP_WRITE);
@@ -2747,10 +2747,10 @@ blk_status_t btrfs_submit_read_repair(struct inode *inode,
                int ret;
                unsigned int offset = i * sectorsize;
 
-               if (!(error_bitmap & (1 << i))) {
+               if (!(error_bitmap & (1U << i))) {
                        struct extent_state *cached = NULL;
 
-                       /* This sector has no error, just finish the read. */
+                       /* This sector has no error, just finish the read */
                        end_page_read(page, true, start + offset, sectorsize);
                        set_extent_uptodate(&BTRFS_I(inode)->io_tree,
                                        start + offset,
---

The division can be replaced by shift as we have it in fs_info and "1U"
in shifts is for clarity that it's performed on unsigned type.