* [PATCH] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp()
@ 2022-09-21 22:10 Zach O'Keefe
2022-09-21 22:39 ` Zach O'Keefe
0 siblings, 1 reply; 3+ messages in thread
From: Zach O'Keefe @ 2022-09-21 22:10 UTC (permalink / raw)
To: Andrew Morton; +Cc: Yang Shi, linux-mm, Zach O'Keefe
By the time we lock a page in collapse_pte_mapped_thp(), the page
mapped by the address pushed onto the slot's .pte_mapped_thp[] array
might have changed arbitrarily since we last looked at it. We
revalidate that the page is still the head of a compound page, but we
don't revalidate if the compound page is of order HPAGE_PMD_ORDER before
applying rmap and page table updates.
Since the kernel now supports large folios of arbitrary order, and since
replacing page's pte mappings by a pmd mapping only makes sense for
compound pages of order HPAGE_PMD_ORDER, revalidate that the compound
order is indeed of order HPAGE_PMD_ORDER before proceeding.
Suggested-by: Yang Shi <shy828301@gmail.com>
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
Andrew, could you please take this into mm-unstable as a prerequisite
patch for the "mm: add file/shmem support to MADV_COLLAPSE" series?
Thank you.
mm/khugepaged.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 122cb72435e3..30f35d646f11 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1448,6 +1448,11 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
goto drop_hpage;
}
+ if (compound_order(hpage) != HPAGE_PMD_ORDER) {
+ result = SCAN_PAGE_COMPOUND;
+ goto drop_hpage;
+ }
+
result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
switch (result) {
case SCAN_SUCCEED:
--
2.37.3.998.g577e59143f-goog
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp()
2022-09-21 22:10 [PATCH] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp() Zach O'Keefe
@ 2022-09-21 22:39 ` Zach O'Keefe
2022-09-22 22:17 ` Zach O'Keefe
0 siblings, 1 reply; 3+ messages in thread
From: Zach O'Keefe @ 2022-09-21 22:39 UTC (permalink / raw)
To: Andrew Morton; +Cc: Yang Shi, linux-mm
On Wed, Sep 21, 2022 at 3:10 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> By the time we lock a page in collapse_pte_mapped_thp(), the page
> mapped by the address pushed onto the slot's .pte_mapped_thp[] array
> might have changed arbitrarily since we last looked at it. We
> revalidate that the page is still the head of a compound page, but we
> don't revalidate if the compound page is of order HPAGE_PMD_ORDER before
> applying rmap and page table updates.
>
> Since the kernel now supports large folios of arbitrary order, and since
> replacing page's pte mappings by a pmd mapping only makes sense for
> compound pages of order HPAGE_PMD_ORDER, revalidate that the compound
> order is indeed of order HPAGE_PMD_ORDER before proceeding.
>
> Suggested-by: Yang Shi <shy828301@gmail.com>
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> ---
>
> Andrew, could you please take this into mm-unstable as a prerequisite
> patch for the "mm: add file/shmem support to MADV_COLLAPSE" series?
> Thank you.
>
> mm/khugepaged.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 122cb72435e3..30f35d646f11 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1448,6 +1448,11 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> goto drop_hpage;
> }
>
> + if (compound_order(hpage) != HPAGE_PMD_ORDER) {
> + result = SCAN_PAGE_COMPOUND;
> + goto drop_hpage;
> + }
> +
> result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
> switch (result) {
> case SCAN_SUCCEED:
> --
> 2.37.3.998.g577e59143f-goog
>
Apologies, I forgot the relevant Link tag:
Link: https://lore.kernel.org/linux-mm/CAHbLzkon+2ky8v9ywGcsTUgXM_B35jt5NThYqQKXW2YV_GUacw@mail.gmail.com/
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp()
2022-09-21 22:39 ` Zach O'Keefe
@ 2022-09-22 22:17 ` Zach O'Keefe
0 siblings, 0 replies; 3+ messages in thread
From: Zach O'Keefe @ 2022-09-22 22:17 UTC (permalink / raw)
To: Andrew Morton; +Cc: Yang Shi, linux-mm
On Wed, Sep 21, 2022 at 3:39 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Wed, Sep 21, 2022 at 3:10 PM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > By the time we lock a page in collapse_pte_mapped_thp(), the page
> > mapped by the address pushed onto the slot's .pte_mapped_thp[] array
> > might have changed arbitrarily since we last looked at it. We
> > revalidate that the page is still the head of a compound page, but we
> > don't revalidate if the compound page is of order HPAGE_PMD_ORDER before
> > applying rmap and page table updates.
> >
> > Since the kernel now supports large folios of arbitrary order, and since
> > replacing page's pte mappings by a pmd mapping only makes sense for
> > compound pages of order HPAGE_PMD_ORDER, revalidate that the compound
> > order is indeed of order HPAGE_PMD_ORDER before proceeding.
> >
> > Suggested-by: Yang Shi <shy828301@gmail.com>
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > ---
> >
> > Andrew, could you please take this into mm-unstable as a prerequisite
> > patch for the "mm: add file/shmem support to MADV_COLLAPSE" series?
> > Thank you.
> >
> > mm/khugepaged.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 122cb72435e3..30f35d646f11 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1448,6 +1448,11 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
> > goto drop_hpage;
> > }
> >
> > + if (compound_order(hpage) != HPAGE_PMD_ORDER) {
> > + result = SCAN_PAGE_COMPOUND;
> > + goto drop_hpage;
> > + }
> > +
> > result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
> > switch (result) {
> > case SCAN_SUCCEED:
> > --
> > 2.37.3.998.g577e59143f-goog
> >
>
> Apologies, I forgot the relevant Link tag:
>
> Link: https://lore.kernel.org/linux-mm/CAHbLzkon+2ky8v9ywGcsTUgXM_B35jt5NThYqQKXW2YV_GUacw@mail.gmail.com/
Apologies x2 ; I mentioned this was a prerequisite for the "mm: add
file/shmem support to MADV_COLLAPSE" series, but it's based on the
current mm-unstable (which includes said series, and thus doesn't even
compile). I'll send a v2 of this patch emminitely, along with a v4 of
the "mm: add file/shmem support to MADV_COLLAPSE" series - applied in
the correct order.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-09-21 22:10 [PATCH] mm/khugepaged: check compound_order() in collapse_pte_mapped_thp() Zach O'Keefe
2022-09-21 22:39 ` Zach O'Keefe
2022-09-22 22:17 ` Zach O'Keefe
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