linux-kernel - Re: [RFC v2 PATCH] mm: shmem: make stat.st_blksize return huge page size if THP is on

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Date:   Sun, 22 Apr 2018 18:47:48 -0600
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        hch@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH] mm: shmem: make stat.st_blksize return huge page
 size if THP is on

On Sat 21-04-18 00:33:59, Yang Shi wrote:
> Since tmpfs THP was supported in 4.8, hugetlbfs is not the only
> filesystem with huge page support anymore. tmpfs can use huge page via
> THP when mounting by "huge=" mount option.
> 
> When applications use huge page on hugetlbfs, it just need check the
> filesystem magic number, but it is not enough for tmpfs. Make
> stat.st_blksize return huge page size if it is mounted by appropriate
> "huge=" option.
> 
> Some applications could benefit from this change, for example QEMU.
> When use mmap file as guest VM backend memory, QEMU typically mmap the
> file size plus one extra page. If the file is on hugetlbfs the extra
> page is huge page size (i.e. 2MB), but it is still 4KB on tmpfs even
> though THP is enabled. tmpfs THP requires VMA is huge page aligned, so
> if 4KB page is used THP will not be used at all. The below /proc/meminfo
> fragment shows the THP use of QEMU with 4K page:
> 
> ShmemHugePages:   679936 kB
> ShmemPmdMapped:        0 kB
> 
> By reading st_blksize, tmpfs can use huge page, then /proc/meminfo looks
> like:
> 
> ShmemHugePages:    77824 kB
> ShmemPmdMapped:     6144 kB
> 
> statfs.f_bsize still returns 4KB for tmpfs since THP could be split, and it
> also may fallback to 4KB page silently if there is not enough huge page.
> Furthermore, different f_bsize makes max_blocks and free_blocks
> calculation harder but without too much benefit. Returning huge page
> size via stat.st_blksize sounds good enough.

I am not sure I understand the above. So does QEMU or other tmpfs users
rely on f_bsize to do mmap alignment tricks? Also I thought that THP
will be used on the first aligned address even when the initial/last
portion of the mapping is not THP aligned.

And more importantly
[...]
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -39,6 +39,7 @@
>  #include <asm/tlbflush.h> /* for arch/microblaze update_mmu_cache() */
>  
>  static struct vfsmount *shm_mnt;
> +static bool is_huge = false;
>  
>  #ifdef CONFIG_SHMEM
>  /*
> @@ -995,6 +996,8 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>  		spin_unlock_irq(&info->lock);
>  	}
>  	generic_fillattr(inode, stat);
> +	if (is_huge)
> +		stat->blksize = HPAGE_PMD_SIZE;
>  	return 0;
>  }
>  
> @@ -3574,6 +3577,7 @@ static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
>  					huge != SHMEM_HUGE_NEVER)
>  				goto bad_val;
>  			sbinfo->huge = huge;
> +			is_huge = true;

Huh! How come this is a global flag. What if we have multiple shmem
mounts some with huge pages enabled and some without? Btw. we seem to
already have that information stored in the supperblock
		} else if (!strcmp(this_char, "huge")) {
			int huge;
			huge = shmem_parse_huge(value);
			if (huge < 0)
				goto bad_val;
			if (!has_transparent_hugepage() &&
					huge != SHMEM_HUGE_NEVER)
				goto bad_val;
			sbinfo->huge = huge;
-- 
Michal Hocko
SUSE Labs