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
