On Thu, 22 Oct 2020, Rik van Riel wrote:
> The allocation flags of anonymous transparent huge pages can be controlled
> through the files in /sys/kernel/mm/transparent_hugepage/defrag, which can
> help the system from getting bogged down in the page reclaim and compaction
> code when many THPs are getting allocated simultaneously.
>
> However, the gfp_mask for shmem THP allocations were not limited by those
> configuration settings, and some workloads ended up with all CPUs stuck
> on the LRU lock in the page reclaim code, trying to allocate dozens of
> THPs simultaneously.
>
> This patch applies the same configurated limitation of THPs to shmem
> hugepage allocations, to prevent that from happening.
>
> This way a THP defrag setting of "never" or "defer+madvise" will result
> in quick allocation failures without direct reclaim when no 2MB free
> pages are available.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
NAK in its present untested form: see below.
I'm open to change here, particularly to Yu Xu's point (in other mail)
about direct reclaim - we avoid that here in Google too: though it's
not so much to avoid the direct reclaim, as to avoid the latencies of
direct compaction, which __GFP_DIRECT_RECLAIM allows as a side-effect.
> ---
> v2: move gfp calculation to shmem_getpage_gfp as suggested by Yu Xu
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index c603237e006c..0a5b164a26d9 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -614,6 +614,8 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask);
> extern void pm_restrict_gfp_mask(void);
> extern void pm_restore_gfp_mask(void);
>
> +extern gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma);
> +
> #ifdef CONFIG_PM_SLEEP
> extern bool pm_suspended_storage(void);
> #else
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9474dbc150ed..9b08ce5cc387 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -649,7 +649,7 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> * available
> * never: never stall for any thp allocation
> */
> -static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
> +gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
> {
> const bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 537c137698f8..9710b9df91e9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1545,8 +1545,8 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
> return NULL;
>
> shmem_pseudo_vma_init(&pvma, info, hindex);
> - page = alloc_pages_vma(gfp | __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN,
> - HPAGE_PMD_ORDER, &pvma, 0, numa_node_id(), true);
> + page = alloc_pages_vma(gfp, HPAGE_PMD_ORDER, &pvma, 0, numa_node_id(),
> + true);
Commendably neat so far.
> shmem_pseudo_vma_destroy(&pvma);
> if (page)
> prep_transhuge_page(page);
> @@ -1802,6 +1802,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> struct page *page;
> enum sgp_type sgp_huge = sgp;
> pgoff_t hindex = index;
> + gfp_t huge_gfp;
> int error;
> int once = 0;
> int alloced = 0;
> @@ -1887,7 +1888,8 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> }
>
> alloc_huge:
> - page = shmem_alloc_and_acct_page(gfp, inode, index, true);
> + huge_gfp = alloc_hugepage_direct_gfpmask(vma);
Still looks nice: but what about the crash when vma is NULL?
It may work for shmem_fault() (though I'll probably disagree on the
details): but tmpfs is a filesystem, so most if not all of the system
calls which arrive here have no vma to offer.
Michal is right to remember pushback before, because tmpfs is a
filesystem, and "huge=" is a mount option: in using a huge=always
filesystem, the user has already declared a preference for huge pages.
Whereas the original anon THP had to deduce that preference from sys
tunables and vma madvice.
I certainly found it a lot easier to ignore all the shifting sandmaze
of the anon THP tunables, and I think Kirill followed me on that.
But it's likely that they have accumulated some defrag wisdom, which
tmpfs can take on board - but please accept that in using a huge mount,
the preference for huge has already been expressed, so I don't expect
anon THP alloc_hugepage_direct_gfpmask() choices will map one to one.
> + page = shmem_alloc_and_acct_page(huge_gfp, inode, index, true);
> if (IS_ERR(page)) {
> alloc_nohuge:
> page = shmem_alloc_and_acct_page(gfp, inode,
>
Hugh
