Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

Darrick J. Wong
2024-07-09T16:50:57Z

On Tue, Jul 09, 2024 at 04:29:07PM +0000, Pankaj Raghav (Samsung) wrote:
> For now, this is the only patch that is blocking for the next version.
>
> Based on the discussion, is the following logical @ryan, @dave and
> @willy?
>
> - We give explicit VM_WARN_ONCE if we try to set folio order range if
> the THP is disabled, min and max is greater than MAX_PAGECACHE_ORDER.
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 14e1415f7dcf4..313c9fad61859 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -394,13 +394,24 @@ static inline void mapping_set_folio_order_range(struct address_space *mapping,
> unsigned int min,
> unsigned int max)
> {
> - if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> + if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> + VM_WARN_ONCE(1,
> + "THP needs to be enabled to support mapping folio order range");
> return;
> + }
>
> - if (min > MAX_PAGECACHE_ORDER)
> + if (min > MAX_PAGECACHE_ORDER) {
> + VM_WARN_ONCE(1,
> + "min order > MAX_PAGECACHE_ORDER. Setting min_order to MAX_PAGECACHE_ORDER");
> min = MAX_PAGECACHE_ORDER;
> - if (max > MAX_PAGECACHE_ORDER)
> + }
> +
> + if (max > MAX_PAGECACHE_ORDER) {
> + VM_WARN_ONCE(1,
> + "max order > MAX_PAGECACHE_ORDER. Setting max_order to MAX_PAGECACHE_ORDER");
> max = MAX_PAGECACHE_ORDER;
> + }
> +
> if (max < min)
> max = min;
>
> - We make THP an explicit dependency for XFS:
>
> diff --git a/fs/xfs/Kconfig b/fs/xfs/Kconfig
> index d41edd30388b7..be2c1c0e9fe8b 100644
> --- a/fs/xfs/Kconfig
> +++ b/fs/xfs/Kconfig
> @@ -5,6 +5,7 @@ config XFS_FS
> select EXPORTFS
> select LIBCRC32C
> select FS_IOMAP
> + select TRANSPARENT_HUGEPAGE
> help
> XFS is a high performance journaling filesystem which originated
> on the SGI IRIX platform. It is completely multi-threaded, can
>
> OR
>
> We create a helper in page cache that FSs can use to check if a specific
> order can be supported at mount time:

I like this solution better; if XFS is going to drop support for o[ld]d
architectures I think we need /some/ sort of notice period. Or at least
a better story than "we want to support 64k fsblocks on x64 so we're
withdrawing support even for 4k fsblocks and smallish filesystems on
m68k".

You probably don't want bs>ps support to block on some arcane discussion
about 32-bit, right? ;)

> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 14e1415f7dcf..9be775ef11a5 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -374,6 +374,14 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
> #define MAX_XAS_ORDER (XA_CHUNK_SHIFT * 2 - 1)
> #define MAX_PAGECACHE_ORDER min(MAX_XAS_ORDER, PREFERRED_MAX_PAGECACHE_ORDER)
>
> +
> +static inline unsigned int mapping_max_folio_order_supported()
> +{
> + if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> + return 0;

Shouldn't this line be indented by two tabs, not six spaces?

> + return MAX_PAGECACHE_ORDER;
> +}

Alternately, should this return the max folio size in bytes?

static inline size_t mapping_max_folio_size(void)
{
if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
return 1U << (PAGE_SHIFT + MAX_PAGECACHE_ORDER);
return PAGE_SIZE;
}

Then the validation looks like:

const size_t max_folio_size = mapping_max_folio_size();

if (mp->m_sb.sb_blocksize > max_folio_size) {
xfs_warn(mp,
"block size (%u bytes) not supported; maximum folio size is %u.",
mp->m_sb.sb_blocksize, max_folio_size);
error = -ENOSYS;
goto out_free_sb;
}

(Don't mind me bikeshedding here.)

> +
>
>
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index b8a93a8f35cac..e2be8743c2c20 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1647,6 +1647,15 @@ xfs_fs_fill_super(
> goto out_free_sb;
> }
>
> + if (mp->m_sb.sb_blocklog - PAGE_SHIFT >
> + mapping_max_folio_order_supported()) {
> + xfs_warn(mp,
> +"Block Size (%d bytes) is not supported. Check MAX_PAGECACHE_ORDER",
> + mp->m_sb.sb_blocksize);

You might as well print MAX_PAGECACHE_ORDER here to make analysis
easier on less-familiar architectures:

xfs_warn(mp,
"block size (%d bytes) is not supported; max folio size is %u.",
mp->m_sb.sb_blocksize,
1U << mapping_max_folio_order_supported());

(I wrote this comment first.)

--D

> + error = -ENOSYS;
> + goto out_free_sb;
> + }
> +
> xfs_warn(mp,
> "EXPERIMENTAL: V5 Filesystem with Large Block Size (%d bytes) enabled.",
> mp->m_sb.sb_blocksize);
>
>
> --
> Pankaj