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Message-ID: <n3jvaazkla3usq5vx4kxsfkr33d2mwm4eu7xpgf7qssktmjwgu@btxoicdj3vrr>
Date: Wed, 5 Nov 2025 10:18:45 +0100
From: Jan Kara <jack@...e.cz>
To: libaokun@...weicloud.com
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
jack@...e.cz, linux-kernel@...r.kernel.org, kernel@...kajraghav.com,
mcgrof@...nel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
yi.zhang@...wei.com, yangerkun@...wei.com, chengzhihao1@...wei.com,
libaokun1@...wei.com
Subject: Re: [PATCH 13/25] ext4: support large block size in
ext4_mb_init_cache()
On Sat 25-10-25 11:22:09, libaokun@...weicloud.com wrote:
> From: Baokun Li <libaokun1@...wei.com>
>
> Currently, ext4_mb_init_cache() uses blocks_per_page to calculate the
> folio index and offset. However, when blocksize is larger than PAGE_SIZE,
> blocks_per_page becomes zero, leading to a potential division-by-zero bug.
>
> Since we now have the folio, we know its exact size. This allows us to
> convert {blocks, groups}_per_page to {blocks, groups}_per_folio, thus
> supporting block sizes greater than page size.
>
> Signed-off-by: Baokun Li <libaokun1@...wei.com>
> Reviewed-by: Zhang Yi <yi.zhang@...wei.com>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> fs/ext4/mballoc.c | 44 ++++++++++++++++++++------------------------
> 1 file changed, 20 insertions(+), 24 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index d42d768a705a..31f4c7d65eb4 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1329,26 +1329,25 @@ static void mb_regenerate_buddy(struct ext4_buddy *e4b)
> * block bitmap and buddy information. The information are
> * stored in the inode as
> *
> - * { page }
> + * { folio }
> * [ group 0 bitmap][ group 0 buddy] [group 1][ group 1]...
> *
> *
> * one block each for bitmap and buddy information.
> - * So for each group we take up 2 blocks. A page can
> - * contain blocks_per_page (PAGE_SIZE / blocksize) blocks.
> - * So it can have information regarding groups_per_page which
> - * is blocks_per_page/2
> + * So for each group we take up 2 blocks. A folio can
> + * contain blocks_per_folio (folio_size / blocksize) blocks.
> + * So it can have information regarding groups_per_folio which
> + * is blocks_per_folio/2
> *
> * Locking note: This routine takes the block group lock of all groups
> - * for this page; do not hold this lock when calling this routine!
> + * for this folio; do not hold this lock when calling this routine!
> */
> -
> static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
> {
> ext4_group_t ngroups;
> unsigned int blocksize;
> - int blocks_per_page;
> - int groups_per_page;
> + int blocks_per_folio;
> + int groups_per_folio;
> int err = 0;
> int i;
> ext4_group_t first_group, group;
> @@ -1365,27 +1364,24 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
> sb = inode->i_sb;
> ngroups = ext4_get_groups_count(sb);
> blocksize = i_blocksize(inode);
> - blocks_per_page = PAGE_SIZE / blocksize;
> + blocks_per_folio = folio_size(folio) / blocksize;
> + WARN_ON_ONCE(!blocks_per_folio);
> + groups_per_folio = DIV_ROUND_UP(blocks_per_folio, 2);
>
> mb_debug(sb, "init folio %lu\n", folio->index);
>
> - groups_per_page = blocks_per_page >> 1;
> - if (groups_per_page == 0)
> - groups_per_page = 1;
> -
> /* allocate buffer_heads to read bitmaps */
> - if (groups_per_page > 1) {
> - i = sizeof(struct buffer_head *) * groups_per_page;
> + if (groups_per_folio > 1) {
> + i = sizeof(struct buffer_head *) * groups_per_folio;
> bh = kzalloc(i, gfp);
> if (bh == NULL)
> return -ENOMEM;
> } else
> bh = &bhs;
>
> - first_group = folio->index * blocks_per_page / 2;
> -
> /* read all groups the folio covers into the cache */
> - for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
> + first_group = EXT4_P_TO_LBLK(inode, folio->index) / 2;
> + for (i = 0, group = first_group; i < groups_per_folio; i++, group++) {
> if (group >= ngroups)
> break;
>
> @@ -1393,7 +1389,7 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
> if (!grinfo)
> continue;
> /*
> - * If page is uptodate then we came here after online resize
> + * If folio is uptodate then we came here after online resize
> * which added some new uninitialized group info structs, so
> * we must skip all initialized uptodate buddies on the folio,
> * which may be currently in use by an allocating task.
> @@ -1413,7 +1409,7 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
> }
>
> /* wait for I/O completion */
> - for (i = 0, group = first_group; i < groups_per_page; i++, group++) {
> + for (i = 0, group = first_group; i < groups_per_folio; i++, group++) {
> int err2;
>
> if (!bh[i])
> @@ -1423,8 +1419,8 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
> err = err2;
> }
>
> - first_block = folio->index * blocks_per_page;
> - for (i = 0; i < blocks_per_page; i++) {
> + first_block = EXT4_P_TO_LBLK(inode, folio->index);
> + for (i = 0; i < blocks_per_folio; i++) {
> group = (first_block + i) >> 1;
> if (group >= ngroups)
> break;
> @@ -1501,7 +1497,7 @@ static int ext4_mb_init_cache(struct folio *folio, char *incore, gfp_t gfp)
>
> out:
> if (bh) {
> - for (i = 0; i < groups_per_page; i++)
> + for (i = 0; i < groups_per_folio; i++)
> brelse(bh[i]);
> if (bh != &bhs)
> kfree(bh);
> --
> 2.46.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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