Re: [PATCH 12/31] huge tmpfs: extend get_user_pages_fast to shmem pmd

[Ingo Molnar <mingo@kernel.org>]

* Hugh Dickins <hughd@google.com> wrote:

> The arch-specific get_user_pages_fast() has a gup_huge_pmd() designed to
> optimize the refcounting on anonymous THP and hugetlbfs pages, with one
> atomic addition to compound head's common refcount.  That optimization
> must be avoided on huge tmpfs team pages, which use normal separate page
> refcounting.  We could combine the PageTeam and PageCompound cases into
> a single simple loop, but would lose the compound optimization that way.
> 
> One cannot go through these functions without wondering why some arches
> (x86, mips) like to SetPageReferenced, while the rest do not: an x86
> optimization that missed being propagated to the other architectures?
> No, see commit 8ee53820edfd ("thp: mmu_notifier_test_young"): it's a
> KVM GRU EPT thing, maybe not useful beyond x86.  I've just followed
> the established practice in each architecture.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> Cc'ed to arch maintainers as an FYI: this patch is not expected to
> go into the tree in the next few weeks, and depends upon a PageTeam
> definition not yet available outside this huge tmpfs patchset.
> Please refer to linux-mm or linux-kernel for more context.
> 
>  arch/mips/mm/gup.c  |   15 ++++++++++++++-
>  arch/s390/mm/gup.c  |   19 ++++++++++++++++++-
>  arch/sparc/mm/gup.c |   19 ++++++++++++++++++-
>  arch/x86/mm/gup.c   |   15 ++++++++++++++-
>  mm/gup.c            |   19 ++++++++++++++++++-
>  5 files changed, 82 insertions(+), 5 deletions(-)
> 
> --- a/arch/mips/mm/gup.c
> +++ b/arch/mips/mm/gup.c
> @@ -81,9 +81,22 @@ static int gup_huge_pmd(pmd_t pmd, unsig
>  	VM_BUG_ON(pte_special(pte));
>  	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  
> -	refs = 0;
>  	head = pte_page(pte);
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +
> +	if (PageTeam(head)) {
> +		/* Handle a huge tmpfs team with normal refcounting. */
> +		do {
> +			get_page(page);
> +			SetPageReferenced(page);
> +			pages[*nr] = page;
> +			(*nr)++;
> +			page++;
> +		} while (addr += PAGE_SIZE, addr != end);
> +		return 1;
> +	}
> +
> +	refs = 0;
>  	do {
>  		VM_BUG_ON(compound_head(page) != head);
>  		pages[*nr] = page;
> --- a/arch/s390/mm/gup.c
> +++ b/arch/s390/mm/gup.c
> @@ -66,9 +66,26 @@ static inline int gup_huge_pmd(pmd_t *pm
>  		return 0;
>  	VM_BUG_ON(!pfn_valid(pmd_val(pmd) >> PAGE_SHIFT));
>  
> -	refs = 0;
>  	head = pmd_page(pmd);
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +
> +	if (PageTeam(head)) {
> +		/* Handle a huge tmpfs team with normal refcounting. */
> +		do {
> +			if (!page_cache_get_speculative(page))
> +				return 0;
> +			if (unlikely(pmd_val(pmd) != pmd_val(*pmdp))) {
> +				put_page(page);
> +				return 0;
> +			}
> +			pages[*nr] = page;
> +			(*nr)++;
> +			page++;
> +		} while (addr += PAGE_SIZE, addr != end);
> +		return 1;
> +	}
> +
> +	refs = 0;
>  	do {
>  		VM_BUG_ON(compound_head(page) != head);
>  		pages[*nr] = page;
> --- a/arch/sparc/mm/gup.c
> +++ b/arch/sparc/mm/gup.c
> @@ -77,9 +77,26 @@ static int gup_huge_pmd(pmd_t *pmdp, pmd
>  	if (write && !pmd_write(pmd))
>  		return 0;
>  
> -	refs = 0;
>  	head = pmd_page(pmd);
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +
> +	if (PageTeam(head)) {
> +		/* Handle a huge tmpfs team with normal refcounting. */
> +		do {
> +			if (!page_cache_get_speculative(page))
> +				return 0;
> +			if (unlikely(pmd_val(pmd) != pmd_val(*pmdp))) {
> +				put_page(page);
> +				return 0;
> +			}
> +			pages[*nr] = page;
> +			(*nr)++;
> +			page++;
> +		} while (addr += PAGE_SIZE, addr != end);
> +		return 1;
> +	}
> +
> +	refs = 0;
>  	do {
>  		VM_BUG_ON(compound_head(page) != head);
>  		pages[*nr] = page;
> --- a/arch/x86/mm/gup.c
> +++ b/arch/x86/mm/gup.c
> @@ -196,9 +196,22 @@ static noinline int gup_huge_pmd(pmd_t p
>  	/* hugepages are never "special" */
>  	VM_BUG_ON(pmd_flags(pmd) & _PAGE_SPECIAL);
>  
> -	refs = 0;
>  	head = pmd_page(pmd);
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +
> +	if (PageTeam(head)) {
> +		/* Handle a huge tmpfs team with normal refcounting. */
> +		do {
> +			get_page(page);
> +			SetPageReferenced(page);
> +			pages[*nr] = page;
> +			(*nr)++;
> +			page++;
> +		} while (addr += PAGE_SIZE, addr != end);
> +		return 1;
> +	}
> +
> +	refs = 0;
>  	do {
>  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
>  		pages[*nr] = page;
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1247,9 +1247,26 @@ static int gup_huge_pmd(pmd_t orig, pmd_
>  	if (write && !pmd_write(orig))
>  		return 0;
>  
> -	refs = 0;
>  	head = pmd_page(orig);
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +
> +	if (PageTeam(head)) {
> +		/* Handle a huge tmpfs team with normal refcounting. */
> +		do {
> +			if (!page_cache_get_speculative(page))
> +				return 0;
> +			if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> +				put_page(page);
> +				return 0;
> +			}
> +			pages[*nr] = page;
> +			(*nr)++;
> +			page++;
> +		} while (addr += PAGE_SIZE, addr != end);
> +		return 1;
> +	}
> +
> +	refs = 0;
>  	do {
>  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
>  		pages[*nr] = page;

Ouch!

Looks like there are two main variants - so these kinds of repetitive patterns 
very much call for some sort of factoring out of common code, right?

Then the fix could be applied to the common portion(s) only, which will cut down 
this gigantic diffstat:

  >  5 files changed, 82 insertions(+), 5 deletions(-)

Thanks,

	Ingo