In preparation for extending vma_alloc_zeroed_movable_folio() to
allocate a arbitrary order folio, expose clear_huge_page()
unconditionally, so that it can be used to zero the allocated folio in
the generic implementation of vma_alloc_zeroed_movable_folio().
Signed-off-by: Ryan Roberts <[email protected]>
---
include/linux/mm.h | 3 ++-
mm/memory.c | 2 +-
2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7f1741bd870a..7e3bf45e6491 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3684,10 +3684,11 @@ enum mf_action_page_type {
*/
extern const struct attribute_group memory_failure_attr_group;
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
extern void clear_huge_page(struct page *page,
unsigned long addr_hint,
unsigned int pages_per_huge_page);
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
int copy_user_large_folio(struct folio *dst, struct folio *src,
unsigned long addr_hint,
struct vm_area_struct *vma);
diff --git a/mm/memory.c b/mm/memory.c
index fb30f7523550..3d4ea668c4d1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5741,7 +5741,6 @@ void __might_fault(const char *file, int line)
EXPORT_SYMBOL(__might_fault);
#endif
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
/*
* Process all subpages of the specified huge page with the specified
* operation. The target subpage will be processed last to keep its
@@ -5839,6 +5838,7 @@ void clear_huge_page(struct page *page,
process_huge_page(addr_hint, pages_per_huge_page, clear_subpage, page);
}
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
static int copy_user_gigantic_page(struct folio *dst, struct folio *src,
unsigned long addr,
struct vm_area_struct *vma,
--
2.25.1
On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
>
> In preparation for extending vma_alloc_zeroed_movable_folio() to
> allocate a arbitrary order folio, expose clear_huge_page()
> unconditionally, so that it can be used to zero the allocated folio in
> the generic implementation of vma_alloc_zeroed_movable_folio().
>
> Signed-off-by: Ryan Roberts <[email protected]>
> ---
> include/linux/mm.h | 3 ++-
> mm/memory.c | 2 +-
> 2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7f1741bd870a..7e3bf45e6491 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
> */
> extern const struct attribute_group memory_failure_attr_group;
>
> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> extern void clear_huge_page(struct page *page,
> unsigned long addr_hint,
> unsigned int pages_per_huge_page);
> +
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
We might not want to depend on THP eventually. Right now, we still
have to, unless splitting is optional, which seems to contradict
06/10. (deferred_split_folio() is a nop without THP.)
On 27/06/2023 02:55, Yu Zhao wrote:
> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
>>
>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>> allocate a arbitrary order folio, expose clear_huge_page()
>> unconditionally, so that it can be used to zero the allocated folio in
>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>
>> Signed-off-by: Ryan Roberts <[email protected]>
>> ---
>> include/linux/mm.h | 3 ++-
>> mm/memory.c | 2 +-
>> 2 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7f1741bd870a..7e3bf45e6491 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>> */
>> extern const struct attribute_group memory_failure_attr_group;
>>
>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>> extern void clear_huge_page(struct page *page,
>> unsigned long addr_hint,
>> unsigned int pages_per_huge_page);
>> +
>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>
> We might not want to depend on THP eventually. Right now, we still
> have to, unless splitting is optional, which seems to contradict
> 06/10. (deferred_split_folio() is a nop without THP.)
Yes, I agree - for large anon folios to work, we depend on THP. But I don't
think that helps us here.
In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
parameter. So the generic/default version of the function now needs a way to
clear a compound page.
I guess I could do something like:
static inline
struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
unsigned long vaddr, gfp_t gfp, int order)
{
struct folio *folio;
folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
order, vma, vaddr, false);
if (folio) {
#ifdef CONFIG_LARGE_FOLIO
clear_huge_page(&folio->page, vaddr, 1U << order);
#else
BUG_ON(order != 0);
clear_user_highpage(&folio->page, vaddr);
#endif
}
return folio;
}
But that's pretty messy and there's no reason why other users might come along
that pass order != 0 and will be surprised by the BUG_ON.
On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <[email protected]> wrote:
>
> On 27/06/2023 02:55, Yu Zhao wrote:
> > On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
> >>
> >> In preparation for extending vma_alloc_zeroed_movable_folio() to
> >> allocate a arbitrary order folio, expose clear_huge_page()
> >> unconditionally, so that it can be used to zero the allocated folio in
> >> the generic implementation of vma_alloc_zeroed_movable_folio().
> >>
> >> Signed-off-by: Ryan Roberts <[email protected]>
> >> ---
> >> include/linux/mm.h | 3 ++-
> >> mm/memory.c | 2 +-
> >> 2 files changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index 7f1741bd870a..7e3bf45e6491 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
> >> */
> >> extern const struct attribute_group memory_failure_attr_group;
> >>
> >> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >> extern void clear_huge_page(struct page *page,
> >> unsigned long addr_hint,
> >> unsigned int pages_per_huge_page);
> >> +
> >> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >
> > We might not want to depend on THP eventually. Right now, we still
> > have to, unless splitting is optional, which seems to contradict
> > 06/10. (deferred_split_folio() is a nop without THP.)
>
> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
> think that helps us here.
>
> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
> parameter. So the generic/default version of the function now needs a way to
> clear a compound page.
>
> I guess I could do something like:
>
> static inline
> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> unsigned long vaddr, gfp_t gfp, int order)
> {
> struct folio *folio;
>
> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
> order, vma, vaddr, false);
> if (folio) {
> #ifdef CONFIG_LARGE_FOLIO
> clear_huge_page(&folio->page, vaddr, 1U << order);
> #else
> BUG_ON(order != 0);
> clear_user_highpage(&folio->page, vaddr);
> #endif
> }
>
> return folio;
> }
>
> But that's pretty messy and there's no reason why other users might come along
> that pass order != 0 and will be surprised by the BUG_ON.
#ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
long vaddr, int order)
{
// how do_huge_pmd_anonymous_page() allocs and clears
vma_alloc_folio(..., *true*);
}
#else
#define alloc_anon_folio(vma, addr, order)
vma_alloc_zeroed_movable_folio(vma, addr)
#endif
On 27/06/2023 09:29, Yu Zhao wrote:
> On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <[email protected]> wrote:
>>
>> On 27/06/2023 02:55, Yu Zhao wrote:
>>> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
>>>>
>>>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>>>> allocate a arbitrary order folio, expose clear_huge_page()
>>>> unconditionally, so that it can be used to zero the allocated folio in
>>>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>>>
>>>> Signed-off-by: Ryan Roberts <[email protected]>
>>>> ---
>>>> include/linux/mm.h | 3 ++-
>>>> mm/memory.c | 2 +-
>>>> 2 files changed, 3 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 7f1741bd870a..7e3bf45e6491 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>>>> */
>>>> extern const struct attribute_group memory_failure_attr_group;
>>>>
>>>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>> extern void clear_huge_page(struct page *page,
>>>> unsigned long addr_hint,
>>>> unsigned int pages_per_huge_page);
>>>> +
>>>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>
>>> We might not want to depend on THP eventually. Right now, we still
>>> have to, unless splitting is optional, which seems to contradict
>>> 06/10. (deferred_split_folio() is a nop without THP.)
>>
>> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
>> think that helps us here.
>>
>> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
>> parameter. So the generic/default version of the function now needs a way to
>> clear a compound page.
>>
>> I guess I could do something like:
>>
>> static inline
>> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>> unsigned long vaddr, gfp_t gfp, int order)
>> {
>> struct folio *folio;
>>
>> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
>> order, vma, vaddr, false);
>> if (folio) {
>> #ifdef CONFIG_LARGE_FOLIO
>> clear_huge_page(&folio->page, vaddr, 1U << order);
>> #else
>> BUG_ON(order != 0);
>> clear_user_highpage(&folio->page, vaddr);
>> #endif
>> }
>>
>> return folio;
>> }
>>
>> But that's pretty messy and there's no reason why other users might come along
>> that pass order != 0 and will be surprised by the BUG_ON.
>
> #ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
> struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
> long vaddr, int order)
> {
> // how do_huge_pmd_anonymous_page() allocs and clears
> vma_alloc_folio(..., *true*);
This controls the mem allocation policy (see mempolicy.c::vma_alloc_folio()) not
clearing. Clearing is done in __do_huge_pmd_anonymous_page():
clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
> }
> #else
> #define alloc_anon_folio(vma, addr, order)
> vma_alloc_zeroed_movable_folio(vma, addr)
> #endif
Sorry I don't get this at all... If you are suggesting to bypass
vma_alloc_zeroed_movable_folio() entirely for the LARGE_ANON_FOLIO case, I don't
think that works because the arch code adds its own gfp flags there. For
example, arm64 adds __GFP_ZEROTAGS for VM_MTE VMAs.
Perhaps we can do away with an arch-owned vma_alloc_zeroed_movable_folio() and
replace it with a new arch_get_zeroed_movable_gfp_flags() then
alloc_anon_folio() add in those flags?
But I still think the cleanest, simplest change is just to unconditionally
expose clear_huge_page() as I've done it.
On Tue, Jun 27, 2023 at 3:41 AM Ryan Roberts <[email protected]> wrote:
>
> On 27/06/2023 09:29, Yu Zhao wrote:
> > On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <[email protected]> wrote:
> >>
> >> On 27/06/2023 02:55, Yu Zhao wrote:
> >>> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
> >>>>
> >>>> In preparation for extending vma_alloc_zeroed_movable_folio() to
> >>>> allocate a arbitrary order folio, expose clear_huge_page()
> >>>> unconditionally, so that it can be used to zero the allocated folio in
> >>>> the generic implementation of vma_alloc_zeroed_movable_folio().
> >>>>
> >>>> Signed-off-by: Ryan Roberts <[email protected]>
> >>>> ---
> >>>> include/linux/mm.h | 3 ++-
> >>>> mm/memory.c | 2 +-
> >>>> 2 files changed, 3 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>>> index 7f1741bd870a..7e3bf45e6491 100644
> >>>> --- a/include/linux/mm.h
> >>>> +++ b/include/linux/mm.h
> >>>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
> >>>> */
> >>>> extern const struct attribute_group memory_failure_attr_group;
> >>>>
> >>>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >>>> extern void clear_huge_page(struct page *page,
> >>>> unsigned long addr_hint,
> >>>> unsigned int pages_per_huge_page);
> >>>> +
> >>>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> >>>
> >>> We might not want to depend on THP eventually. Right now, we still
> >>> have to, unless splitting is optional, which seems to contradict
> >>> 06/10. (deferred_split_folio() is a nop without THP.)
> >>
> >> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
> >> think that helps us here.
> >>
> >> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
> >> parameter. So the generic/default version of the function now needs a way to
> >> clear a compound page.
> >>
> >> I guess I could do something like:
> >>
> >> static inline
> >> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> >> unsigned long vaddr, gfp_t gfp, int order)
> >> {
> >> struct folio *folio;
> >>
> >> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
> >> order, vma, vaddr, false);
> >> if (folio) {
> >> #ifdef CONFIG_LARGE_FOLIO
> >> clear_huge_page(&folio->page, vaddr, 1U << order);
> >> #else
> >> BUG_ON(order != 0);
> >> clear_user_highpage(&folio->page, vaddr);
> >> #endif
> >> }
> >>
> >> return folio;
> >> }
> >>
> >> But that's pretty messy and there's no reason why other users might come along
> >> that pass order != 0 and will be surprised by the BUG_ON.
> >
> > #ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
> > struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
> > long vaddr, int order)
> > {
> > // how do_huge_pmd_anonymous_page() allocs and clears
> > vma_alloc_folio(..., *true*);
>
> This controls the mem allocation policy (see mempolicy.c::vma_alloc_folio()) not
> clearing. Clearing is done in __do_huge_pmd_anonymous_page():
>
> clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
Sorry for rushing this previously. This is what I meant. The #ifdef
makes it safe to use clear_huge_page() without 01/10. I highlighted
the last parameter to vma_alloc_folio() only because it's different
from what you chose (not implying it clears the folio).
> > }
> > #else
> > #define alloc_anon_folio(vma, addr, order)
> > vma_alloc_zeroed_movable_folio(vma, addr)
> > #endif
>
> Sorry I don't get this at all... If you are suggesting to bypass
> vma_alloc_zeroed_movable_folio() entirely for the LARGE_ANON_FOLIO case
Correct.
> I don't
> think that works because the arch code adds its own gfp flags there. For
> example, arm64 adds __GFP_ZEROTAGS for VM_MTE VMAs.
I think it's the opposite: it should be safer to reuse the THP code because
1. It's an existing case that has been working for PMD_ORDER folios
mapped by PTEs, and it's an arch-independent API which would be easier
to review.
2. Use vma_alloc_zeroed_movable_folio() for large folios is a *new*
case. It's an arch-*dependent* API which I have no idea what VM_MTE
does (should do) to large folios and don't plan to answer that for
now.
> Perhaps we can do away with an arch-owned vma_alloc_zeroed_movable_folio() and
> replace it with a new arch_get_zeroed_movable_gfp_flags() then
> alloc_anon_folio() add in those flags?
>
> But I still think the cleanest, simplest change is just to unconditionally
> expose clear_huge_page() as I've done it.
The fundamental choice there as I see it is to whether the first step
of large anon folios should lean toward the THP code base or the base
page code base (I'm a big fan of the answer "Neither -- we should
create something entirely new instead"). My POV is that the THP code
base would allow us to move faster, since it's proven to work for a
very similar case (PMD_ORDER folios mapped by PTEs).
On 27/06/2023 19:26, Yu Zhao wrote:
> On Tue, Jun 27, 2023 at 3:41 AM Ryan Roberts <[email protected]> wrote:
>>
>> On 27/06/2023 09:29, Yu Zhao wrote:
>>> On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <[email protected]> wrote:
>>>>
>>>> On 27/06/2023 02:55, Yu Zhao wrote:
>>>>> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <[email protected]> wrote:
>>>>>>
>>>>>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>>>>>> allocate a arbitrary order folio, expose clear_huge_page()
>>>>>> unconditionally, so that it can be used to zero the allocated folio in
>>>>>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>>>>>
>>>>>> Signed-off-by: Ryan Roberts <[email protected]>
>>>>>> ---
>>>>>> include/linux/mm.h | 3 ++-
>>>>>> mm/memory.c | 2 +-
>>>>>> 2 files changed, 3 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>>> index 7f1741bd870a..7e3bf45e6491 100644
>>>>>> --- a/include/linux/mm.h
>>>>>> +++ b/include/linux/mm.h
>>>>>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>>>>>> */
>>>>>> extern const struct attribute_group memory_failure_attr_group;
>>>>>>
>>>>>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>>>> extern void clear_huge_page(struct page *page,
>>>>>> unsigned long addr_hint,
>>>>>> unsigned int pages_per_huge_page);
>>>>>> +
>>>>>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>>>
>>>>> We might not want to depend on THP eventually. Right now, we still
>>>>> have to, unless splitting is optional, which seems to contradict
>>>>> 06/10. (deferred_split_folio() is a nop without THP.)
>>>>
>>>> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
>>>> think that helps us here.
>>>>
>>>> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
>>>> parameter. So the generic/default version of the function now needs a way to
>>>> clear a compound page.
>>>>
>>>> I guess I could do something like:
>>>>
>>>> static inline
>>>> struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>>>> unsigned long vaddr, gfp_t gfp, int order)
>>>> {
>>>> struct folio *folio;
>>>>
>>>> folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
>>>> order, vma, vaddr, false);
>>>> if (folio) {
>>>> #ifdef CONFIG_LARGE_FOLIO
>>>> clear_huge_page(&folio->page, vaddr, 1U << order);
>>>> #else
>>>> BUG_ON(order != 0);
>>>> clear_user_highpage(&folio->page, vaddr);
>>>> #endif
>>>> }
>>>>
>>>> return folio;
>>>> }
>>>>
>>>> But that's pretty messy and there's no reason why other users might come along
>>>> that pass order != 0 and will be surprised by the BUG_ON.
>>>
>>> #ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
>>> struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
>>> long vaddr, int order)
>>> {
>>> // how do_huge_pmd_anonymous_page() allocs and clears
>>> vma_alloc_folio(..., *true*);
>>
>> This controls the mem allocation policy (see mempolicy.c::vma_alloc_folio()) not
>> clearing. Clearing is done in __do_huge_pmd_anonymous_page():
>>
>> clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
>
> Sorry for rushing this previously. This is what I meant. The #ifdef
> makes it safe to use clear_huge_page() without 01/10. I highlighted
> the last parameter to vma_alloc_folio() only because it's different
> from what you chose (not implying it clears the folio).>
>>> }
>>> #else
>>> #define alloc_anon_folio(vma, addr, order)
>>> vma_alloc_zeroed_movable_folio(vma, addr)
>>> #endif
>>
>> Sorry I don't get this at all... If you are suggesting to bypass
>> vma_alloc_zeroed_movable_folio() entirely for the LARGE_ANON_FOLIO case
>
> Correct.
>
>> I don't
>> think that works because the arch code adds its own gfp flags there. For
>> example, arm64 adds __GFP_ZEROTAGS for VM_MTE VMAs.
>
> I think it's the opposite: it should be safer to reuse the THP code because
> 1. It's an existing case that has been working for PMD_ORDER folios
> mapped by PTEs, and it's an arch-independent API which would be easier
> to review.
> 2. Use vma_alloc_zeroed_movable_folio() for large folios is a *new*
> case. It's an arch-*dependent* API which I have no idea what VM_MTE
> does (should do) to large folios and don't plan to answer that for
> now.
I've done some archaology on this now, and convinced myself that your suggestion
is a good one - sorry for doubting it!
If you are interested here are the details: Only arm64 and ia64 do something
non-standard in vma_alloc_zeroed_movable_folio(). ia64 flushes the dcache for
the folio - but given it does not support THP this is not a problem for the THP
path. arm64 adds the __GFP_ZEROTAGS flag which means that the MTE tags will be
zeroed at the same time as the page is zeroed. This is a perf optimization - if
its not performed then it will be done at set_pte_at(), which is how this works
for the THP path.
So on that basis, I agree we can use your proposed alloc_anon_folio() approach.
arm64 will lose the MTE optimization but that can be added back later if needed.
So no need to unconditionally expose clear_huge_page() and no need to modify all
the arch vma_alloc_zeroed_movable_folio() implementations.
Thanks,
Ryan
>
>> Perhaps we can do away with an arch-owned vma_alloc_zeroed_movable_folio() and
>> replace it with a new arch_get_zeroed_movable_gfp_flags() then
>> alloc_anon_folio() add in those flags?
>>
>> But I still think the cleanest, simplest change is just to unconditionally
>> expose clear_huge_page() as I've done it.
>
> The fundamental choice there as I see it is to whether the first step
> of large anon folios should lean toward the THP code base or the base
> page code base (I'm a big fan of the answer "Neither -- we should
> create something entirely new instead"). My POV is that the THP code
> base would allow us to move faster, since it's proven to work for a
> very similar case (PMD_ORDER folios mapped by PTEs).