Re: [PATCH] mm,thp,shmem: limit shmem THP alloc gfp_mask

[Yu Xu]

On 10/22/20 11:48 AM, 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>
> ---
>
> 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..d1290eb508e5 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1545,8 +1545,11 @@ 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);
> +	/* Limit the gfp mask according to THP configuration. */
> +	gfp |= __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN;
> +	gfp &= alloc_hugepage_direct_gfpmask(&pvma);

It is fine to reuse `alloc_hugepage_direct_gfpmask`, but
`pvma.vm_flags & VM_HUGEPAGE` is always false here, thus,
`vma_madvised` in `alloc_hugepage_direct_gfpmask` will always
be false.

That is why I chose to do the gfp_mask fixup in `shmem_getpage_gfp`,
using `sgp_huge` to indicate `vma_madvised`, although with some silly
mistakes pointed out by you, in another mail thread.

It will be better if vma_madvised is well handled in your solution.

> +	page = alloc_pages_vma(gfp, HPAGE_PMD_ORDER, &pvma, 0, numa_node_id(),
> +			       true);
>   	shmem_pseudo_vma_destroy(&pvma);
>   	if (page)
>   		prep_transhuge_page(page);

--
Thanks,
Yu