Re: [PATCH] ext4: fix marking group trimmed if all blocks not trimmed

From: tytso
Date: Fri Aug 07 2020 - 18:02:03 EST

On Mon, Aug 03, 2020 at 04:17:44PM +0000, Lazar Beloica wrote:
> When FTRIM is issued on a group, ext4 marks it as trimmed so another FTRIM
> on the same group has no effect. Ext4 marks group as trimmed if at least
> one block is trimmed, therefore it is possible that a group is marked as
> trimmed even if there are blocks in that group left untrimmed.
> This patch marks group as trimmed only if there are no more blocks
> in that group to be trimmed.

This patch makes no sense; first of all, the changes below are *not*
in the function ext4_trim_extent(), but rather ext4_trim_all_free().
It appears that the diff is based off of v5.8-rc2, based on the index
c0a331e, but then I'm not sure how you generated the diff?

Secondly, ext4_trim_all_free(), which is where these two patch hunks

> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index c0a331e..130936b 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -5346,6 +5346,7 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
> {
> void *bitmap;
> ext4_grpblk_t next, count = 0, free_count = 0;
> + ext4_fsblk_t max_blks = ext4_blocks_count(EXT4_SB(sb)->s_es);
> struct ext4_buddy e4b;
> int ret = 0;
> @@ -5401,7 +5402,9 @@ static int ext4_trim_extent(struct super_block *sb, int start, int count,
> if (!ret) {
> ret = count;
> - EXT4_MB_GRP_SET_TRIMMED(e4b.bd_info);
> + next = mb_find_next_bit(bitmap, max_blks, max + 1);
> + if (next == max_blks)
> + EXT4_MB_GRP_SET_TRIMMED(e4b.bd_info);
> }
> out:
> ext4_unlock_group(sb, group);

The function send discards for blocks in a block group which are
freed. So setting max_blks to be ext4_blocks_count() and then using
it as the limit to mb_find_next_bit() makes no sense. First of all
next will never be equal to max_blks, since next is an offset relative
to the beginning of the block group, and max_blks is set number of
blocks in the entire file system.

Secondly, mb_find_next_bit is searching a bitmap, which is a single
file system block (e.g., 4k in a 4k block file system). So if
max_blks is the the number of blocks in (for example) a 10TB file
system, this is going to potentially cause a kernel oops.

How, exactly did you test this patch?

- Ted