follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
target of FOLL_WRITE either. However add the checks.
Namely, either the need to CoW due to missing write bit, or proper
unsharing on !AnonExclusive pages over R/O pins to reject the follow page.
That brings this function closer to follow_hugetlb_page().
So we don't care before, and also for now. But we'll care if we switch
over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
return -EMLINK properly, as that's the gup internal api to mean "we should
unshare". Not really needed for follow page path, though.
When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
clear that it just should never fail. When error happens, instead of
setting page==NULL, capture the errno instead.
Reviewed-by: Mike Kravetz <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
---
mm/hugetlb.c | 31 ++++++++++++++++++++-----------
1 file changed, 20 insertions(+), 11 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f75f5e78ff0b..27367edf5c72 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
struct page *page = NULL;
spinlock_t *ptl;
pte_t *pte, entry;
-
- /*
- * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
- * follow_hugetlb_page().
- */
- if (WARN_ON_ONCE(flags & FOLL_PIN))
- return NULL;
+ int ret;
hugetlb_vma_lock_read(vma);
pte = hugetlb_walk(vma, haddr, huge_page_size(h));
@@ -6478,8 +6472,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
ptl = huge_pte_lock(h, mm, pte);
entry = huge_ptep_get(pte);
if (pte_present(entry)) {
- page = pte_page(entry) +
- ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+ page = pte_page(entry);
+
+ if ((flags & FOLL_WRITE) && !huge_pte_write(entry)) {
+ page = NULL;
+ goto out;
+ }
+
+ if (gup_must_unshare(vma, flags, page)) {
+ /* Tell the caller to do unsharing */
+ page = ERR_PTR(-EMLINK);
+ goto out;
+ }
+
+ page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+
/*
* Note that page may be a sub-page, and with vmemmap
* optimizations the page struct may be read only.
@@ -6489,8 +6496,10 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
* try_grab_page() should always be able to get the page here,
* because we hold the ptl lock and have verified pte_present().
*/
- if (try_grab_page(page, flags)) {
- page = NULL;
+ ret = try_grab_page(page, flags);
+
+ if (WARN_ON_ONCE(ret)) {
+ page = ERR_PTR(ret);
goto out;
}
}
--
2.40.1
On 23.06.23 16:29, Peter Xu wrote:
> follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
> target of FOLL_WRITE either. However add the checks.
>
> Namely, either the need to CoW due to missing write bit, or proper
> unsharing on !AnonExclusive pages over R/O pins to reject the follow page.
> That brings this function closer to follow_hugetlb_page().
>
> So we don't care before, and also for now. But we'll care if we switch
> over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
> return -EMLINK properly, as that's the gup internal api to mean "we should
> unshare". Not really needed for follow page path, though.
>
> When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> clear that it just should never fail. When error happens, instead of
> setting page==NULL, capture the errno instead.
>
> Reviewed-by: Mike Kravetz <[email protected]>
> Signed-off-by: Peter Xu <[email protected]>
> ---
> mm/hugetlb.c | 31 ++++++++++++++++++++-----------
> 1 file changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f75f5e78ff0b..27367edf5c72 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> struct page *page = NULL;
> spinlock_t *ptl;
> pte_t *pte, entry;
> -
> - /*
> - * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> - * follow_hugetlb_page().
> - */
> - if (WARN_ON_ONCE(flags & FOLL_PIN))
> - return NULL;
> + int ret;
>
> hugetlb_vma_lock_read(vma);
> pte = hugetlb_walk(vma, haddr, huge_page_size(h));
> @@ -6478,8 +6472,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> ptl = huge_pte_lock(h, mm, pte);
> entry = huge_ptep_get(pte);
> if (pte_present(entry)) {
> - page = pte_page(entry) +
> - ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> + page = pte_page(entry);
> +
> + if ((flags & FOLL_WRITE) && !huge_pte_write(entry)) {
> + page = NULL;
> + goto out;
> + }
> +
> + if (gup_must_unshare(vma, flags, page)) {
> + /* Tell the caller to do unsharing */
> + page = ERR_PTR(-EMLINK);
> + goto out;
> + }
No need to check if the page is writable (like all other callers and as
gup_must_unshare() documents -- "for which pages that are
write-protected in the page table")
if (!huge_pte_write(entry) && gup_must_unshare(vma, flags, page)) {
With that
Reviewed-by: David Hildenbrand <[email protected]>
--
Cheers,
David / dhildenb
On Mon, Jun 26, 2023 at 10:06:24AM +0200, David Hildenbrand wrote:
> On 23.06.23 16:29, Peter Xu wrote:
> > follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
> > target of FOLL_WRITE either. However add the checks.
> >
> > Namely, either the need to CoW due to missing write bit, or proper
> > unsharing on !AnonExclusive pages over R/O pins to reject the follow page.
> > That brings this function closer to follow_hugetlb_page().
> >
> > So we don't care before, and also for now. But we'll care if we switch
> > over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
> > return -EMLINK properly, as that's the gup internal api to mean "we should
> > unshare". Not really needed for follow page path, though.
> >
> > When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> > clear that it just should never fail. When error happens, instead of
> > setting page==NULL, capture the errno instead.
> >
> > Reviewed-by: Mike Kravetz <[email protected]>
> > Signed-off-by: Peter Xu <[email protected]>
> > ---
> > mm/hugetlb.c | 31 ++++++++++++++++++++-----------
> > 1 file changed, 20 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index f75f5e78ff0b..27367edf5c72 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > struct page *page = NULL;
> > spinlock_t *ptl;
> > pte_t *pte, entry;
> > -
> > - /*
> > - * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> > - * follow_hugetlb_page().
> > - */
> > - if (WARN_ON_ONCE(flags & FOLL_PIN))
> > - return NULL;
> > + int ret;
> > hugetlb_vma_lock_read(vma);
> > pte = hugetlb_walk(vma, haddr, huge_page_size(h));
> > @@ -6478,8 +6472,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > ptl = huge_pte_lock(h, mm, pte);
> > entry = huge_ptep_get(pte);
> > if (pte_present(entry)) {
> > - page = pte_page(entry) +
> > - ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> > + page = pte_page(entry);
> > +
> > + if ((flags & FOLL_WRITE) && !huge_pte_write(entry)) {
> > + page = NULL;
> > + goto out;
> > + }
> > +
> > + if (gup_must_unshare(vma, flags, page)) {
> > + /* Tell the caller to do unsharing */
> > + page = ERR_PTR(-EMLINK);
> > + goto out;
> > + }
>
>
> No need to check if the page is writable (like all other callers and as
> gup_must_unshare() documents -- "for which pages that are write-protected in
> the page table")
>
> if (!huge_pte_write(entry) && gup_must_unshare(vma, flags, page)) {
Sure.
I was wondering whether we should just allow passing in "write" into
gup_must_unshare(), it'll just be a bit weird that it'll return false
directly if write, meanwhile hopefully that makes it easier to be
consistent. I'll leave that as-is for now, anyway.
For this one I'll just merge it into:
if (!huge_pte_write(entry)) {
if (flags & FOLL_WRITE) {
page = NULL;
goto out;
}
if (gup_must_unshare(vma, flags, page)) {
/* Tell the caller to do unsharing */
page = ERR_PTR(-EMLINK);
goto out;
}
}
>
>
> With that
>
> Reviewed-by: David Hildenbrand <[email protected]>
Thanks,
--
Peter Xu