linux-ext4 - Re: [PATCH v6 7/7] ext4: make prefetch_block

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Date:   Fri, 2 Apr 2021 09:46:22 -0700
From:   harshad shirwadkar <harshadshirwadkar@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>
Subject: Re: [PATCH v6 7/7] ext4: make prefetch_block_bitmaps default

Thanks for taking a look!

On Thu, Apr 1, 2021 at 10:16 PM Andreas Dilger <adilger@...ger.ca> wrote:
>
> On Apr 1, 2021, at 11:45, Harshad Shirwadkar <harshadshirwadkar@...il.com> wrote:
> >
> > Block bitmap prefetching is needed for these allocator optimization
> > data structures to get populated and provide better group scanning
> > order. So, turn it on bu default. prefetch_block_bitmaps mount option
> > is now marked as removed and a new option no_prefetch_block_bitmaps is
> > added to disable block bitmap prefetching.
>
> This makes it more difficult to change between an old kernel and a new one
> using this option. It would be better to keep prefetch_block_bitmaps to turn
> the option on (not harmful if it is already on), and no_* turn it off.
How so? This patch doesn't get rid of the prefetch_block_bitmaps mount
option, it just marks it as "removed". So, you can still pass
"prefetch_block_bitmaps" mount option and get the prefetching
behavior, the only difference is that you'll get an additional kernel
message saying that "ignoring removed mount option
prefetch_block_bitmaps", which I thought is good since looking at that
message, users will eventually remove that mount option from their
mount arguments.

Thanks,
Harshad
>
> Cheers, Andreas
>
> > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
> > ---
> > fs/ext4/ext4.h  |  2 +-
> > fs/ext4/super.c | 15 ++++++++-------
> > 2 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 9a5afe9d2310..20c757f711e7 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1227,7 +1227,7 @@ struct ext4_inode_info {
> > #define EXT4_MOUNT_JOURNAL_CHECKSUM    0x800000 /* Journal checksums */
> > #define EXT4_MOUNT_JOURNAL_ASYNC_COMMIT    0x1000000 /* Journal Async Commit */
> > #define EXT4_MOUNT_WARN_ON_ERROR    0x2000000 /* Trigger WARN_ON on error */
> > -#define EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS 0x4000000
> > +#define EXT4_MOUNT_NO_PREFETCH_BLOCK_BITMAPS 0x4000000
> > #define EXT4_MOUNT_DELALLOC        0x8000000 /* Delalloc support */
> > #define EXT4_MOUNT_DATA_ERR_ABORT    0x10000000 /* Abort on file data write */
> > #define EXT4_MOUNT_BLOCK_VALIDITY    0x20000000 /* Block validity checking */
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 6116640081c0..cec0fb07916b 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -1687,7 +1687,7 @@ enum {
> >    Opt_dioread_nolock, Opt_dioread_lock,
> >    Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
> >    Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
> > -    Opt_prefetch_block_bitmaps, Opt_mb_optimize_scan,
> > +    Opt_no_prefetch_block_bitmaps, Opt_mb_optimize_scan,
> > #ifdef CONFIG_EXT4_DEBUG
> >    Opt_fc_debug_max_replay, Opt_fc_debug_force
> > #endif
> > @@ -1787,7 +1787,8 @@ static const match_table_t tokens = {
> >    {Opt_inlinecrypt, "inlinecrypt"},
> >    {Opt_nombcache, "nombcache"},
> >    {Opt_nombcache, "no_mbcache"},    /* for backward compatibility */
> > -    {Opt_prefetch_block_bitmaps, "prefetch_block_bitmaps"},
> > +    {Opt_removed, "prefetch_block_bitmaps"},
> > +    {Opt_no_prefetch_block_bitmaps, "no_prefetch_block_bitmaps"},
> >    {Opt_mb_optimize_scan, "mb_optimize_scan=%d"},
> >    {Opt_removed, "check=none"},    /* mount option from ext2/3 */
> >    {Opt_removed, "nocheck"},    /* mount option from ext2/3 */
> > @@ -2009,7 +2010,7 @@ static const struct mount_opts {
> >    {Opt_max_dir_size_kb, 0, MOPT_GTE0},
> >    {Opt_test_dummy_encryption, 0, MOPT_STRING},
> >    {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
> > -    {Opt_prefetch_block_bitmaps, EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS,
> > +    {Opt_no_prefetch_block_bitmaps, EXT4_MOUNT_NO_PREFETCH_BLOCK_BITMAPS,
> >     MOPT_SET},
> >    {Opt_mb_optimize_scan, EXT4_MOUNT2_MB_OPTIMIZE_SCAN, MOPT_GTE0},
> > #ifdef CONFIG_EXT4_DEBUG
> > @@ -3706,11 +3707,11 @@ static struct ext4_li_request *ext4_li_request_new(struct super_block *sb,
> >
> >    elr->lr_super = sb;
> >    elr->lr_first_not_zeroed = start;
> > -    if (test_opt(sb, PREFETCH_BLOCK_BITMAPS))
> > -        elr->lr_mode = EXT4_LI_MODE_PREFETCH_BBITMAP;
> > -    else {
> > +    if (test_opt(sb, NO_PREFETCH_BLOCK_BITMAPS)) {
> >        elr->lr_mode = EXT4_LI_MODE_ITABLE;
> >        elr->lr_next_group = start;
> > +    } else {
> > +        elr->lr_mode = EXT4_LI_MODE_PREFETCH_BBITMAP;
> >    }
> >
> >    /*
> > @@ -3741,7 +3742,7 @@ int ext4_register_li_request(struct super_block *sb,
> >        goto out;
> >    }
> >
> > -    if (!test_opt(sb, PREFETCH_BLOCK_BITMAPS) &&
> > +    if (test_opt(sb, NO_PREFETCH_BLOCK_BITMAPS) &&
> >        (first_not_zeroed == ngroups || sb_rdonly(sb) ||
> >         !test_opt(sb, INIT_INODE_TABLE)))
> >        goto out;
> > --
> > 2.31.0.291.g576ba9dcdaf-goog
> >