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

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

[Hugh Dickins]

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;

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Hugh Dickins]

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;

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

[Ingo Molnar]

* 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

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

[Hugh Dickins]

On Wed, 6 Apr 2016, Ingo Molnar wrote:
> * Hugh Dickins <hughd@google.com> wrote:
> 
> > ---
> > 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.

Actually, Andrew took it and the rest into mmotm yesterday, to give them
better exposure through linux-next, so they should appear there soon.

> > 
> >  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(-)
...
> 
> Ouch!

Oh sorry, I didn't mean to hurt you ;)

> 
> 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?

Hmm.  I'm still struggling between the two extremes, of

(a) agreeing completely with you, and saying, yeah, I'll take on the job
    of refactoring every architecture's get_user_pages_as_fast_as_you_can(),
    without much likelihood of testing more than one,

and

(b) running a mile, and pointing out that we have a tradition of using
    arch/x86/mm/gup.c as a template for the others, and here I've just
    added a few more lines to that template (which never gets built more
    than once into any kernel).

Both are appealing in their different ways, but I think you can tell
which I'm leaning towards...

Honestly, I am still struggling between those two; but I think the patch
as it stands is one thing, and cleanup for commonality should be another
however weaselly that sounds ("I'll come back to it" - yeah, right).

Hugh

> 
> 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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Hugh Dickins]

On Wed, 6 Apr 2016, Ingo Molnar wrote:
> * Hugh Dickins <hughd@google.com> wrote:
> 
> > ---
> > 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.

Actually, Andrew took it and the rest into mmotm yesterday, to give them
better exposure through linux-next, so they should appear there soon.

> > 
> >  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(-)
...
> 
> Ouch!

Oh sorry, I didn't mean to hurt you ;)

> 
> 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?

Hmm.  I'm still struggling between the two extremes, of

(a) agreeing completely with you, and saying, yeah, I'll take on the job
    of refactoring every architecture's get_user_pages_as_fast_as_you_can(),
    without much likelihood of testing more than one,

and

(b) running a mile, and pointing out that we have a tradition of using
    arch/x86/mm/gup.c as a template for the others, and here I've just
    added a few more lines to that template (which never gets built more
    than once into any kernel).

Both are appealing in their different ways, but I think you can tell
which I'm leaning towards...

Honestly, I am still struggling between those two; but I think the patch
as it stands is one thing, and cleanup for commonality should be another
however weaselly that sounds ("I'll come back to it" - yeah, right).

Hugh

> 
> 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

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

[Ingo Molnar]

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

> > >  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(-)
> ...

> > 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?
> 
> Hmm.  I'm still struggling between the two extremes, of
> 
> (a) agreeing completely with you, and saying, yeah, I'll take on the job
>     of refactoring every architecture's get_user_pages_as_fast_as_you_can(),
>     without much likelihood of testing more than one,
> 
> and
> 
> (b) running a mile, and pointing out that we have a tradition of using
>     arch/x86/mm/gup.c as a template for the others, and here I've just
>     added a few more lines to that template (which never gets built more
>     than once into any kernel).
> 
> Both are appealing in their different ways, but I think you can tell
> which I'm leaning towards...
> 
> Honestly, I am still struggling between those two; but I think the patch
> as it stands is one thing, and cleanup for commonality should be another
> however weaselly that sounds ("I'll come back to it" - yeah, right).

Yeah, so my worry is this: your patch for example roughly doubles the algorithmic 
complexity of mm/gup.c and arch/*/mm/gup.c's ::gup_huge_pmd().

And you want this to add a new feature!

So it really looks like to me this is the last sane chance to unify cheaply, then 
add the feature you want. Everyone else in the future will be able to refer to 
your example to chicken out! ;-)

Thanks,

	Ingo

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Ingo Molnar]

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

> > >  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(-)
> ...

> > 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?
> 
> Hmm.  I'm still struggling between the two extremes, of
> 
> (a) agreeing completely with you, and saying, yeah, I'll take on the job
>     of refactoring every architecture's get_user_pages_as_fast_as_you_can(),
>     without much likelihood of testing more than one,
> 
> and
> 
> (b) running a mile, and pointing out that we have a tradition of using
>     arch/x86/mm/gup.c as a template for the others, and here I've just
>     added a few more lines to that template (which never gets built more
>     than once into any kernel).
> 
> Both are appealing in their different ways, but I think you can tell
> which I'm leaning towards...
> 
> Honestly, I am still struggling between those two; but I think the patch
> as it stands is one thing, and cleanup for commonality should be another
> however weaselly that sounds ("I'll come back to it" - yeah, right).

Yeah, so my worry is this: your patch for example roughly doubles the algorithmic 
complexity of mm/gup.c and arch/*/mm/gup.c's ::gup_huge_pmd().

And you want this to add a new feature!

So it really looks like to me this is the last sane chance to unify cheaply, then 
add the feature you want. Everyone else in the future will be able to refer to 
your example to chicken out! ;-)

Thanks,

	Ingo