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Message-ID: <Yw/2CDbFDX5cnB2o@monkey>
Date: Wed, 31 Aug 2022 17:00:08 -0700
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
inuxppc-dev@...ts.ozlabs.org, linux-ia64@...r.kernel.org,
David Hildenbrand <david@...hat.com>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
Michael Ellerman <mpe@...erman.id.au>,
Muchun Song <songmuchun@...edance.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask
On 08/31/22 09:07, Baolin Wang wrote:
>
>
> On 8/31/2022 2:39 AM, Mike Kravetz wrote:
> > On 08/30/22 09:44, Mike Kravetz wrote:
> > > On 08/30/22 09:06, Baolin Wang wrote:
> > > > Hi Mike,
> > > >
> > > > On 8/30/2022 7:40 AM, Mike Kravetz wrote:
> > > > > During discussions of this series [1], it was suggested that hugetlb
> > > > > handling code in follow_page_mask could be simplified. At the beginning
> > > > > of follow_page_mask, there currently is a call to follow_huge_addr which
> > > > > 'may' handle hugetlb pages. ia64 is the only architecture which provides
> > > > > a follow_huge_addr routine that does not return error. Instead, at each
> > > > > level of the page table a check is made for a hugetlb entry. If a hugetlb
> > > > > entry is found, a call to a routine associated with that entry is made.
> > > > >
> > > > > Currently, there are two checks for hugetlb entries at each page table
> > > > > level. The first check is of the form:
> > > > > if (p?d_huge())
> > > > > page = follow_huge_p?d();
> > > > > the second check is of the form:
> > > > > if (is_hugepd())
> > > > > page = follow_huge_pd().
> > > > >
> > > > > We can replace these checks, as well as the special handling routines
> > > > > such as follow_huge_p?d() and follow_huge_pd() with a single routine to
> > > > > handle hugetlb vmas.
> > > > >
> > > > > A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
> > > > > beginning of follow_page_mask. hugetlb_follow_page_mask will use the
> > > > > existing routine huge_pte_offset to walk page tables looking for hugetlb
> > > > > entries. huge_pte_offset can be overwritten by architectures, and already
> > > > > handles special cases such as hugepd entries.
> > > >
> > > > Could you also mention that this patch will fix the lock issue for
> > > > CONT-PTE/PMD hugetlb by changing to use huge_pte_lock()? which will help
> > > > people to understand the issue.
> > >
> > > Will update message in v2. Thanks for taking a look!
> > >
> >
> > One additional thought, we 'may' need a separate patch to fix the locking
> > issues that can be easily backported. Not sure this 'simplification' is
> > a good backport candidate.
>
> Yes, that was my thought before, but David did not like adding more
> make-legacy-cruft-happy code.
>
> So how about creating a series that contains 3 patches: picking up patch 1
> and patch 3 of my previous series [1], and your current patch? That means
> patch 1 and patch 2 in this series can fix the lock issue explicitly and be
> suitable to backport, meanwhile patch 3 (which is your current patch) will
> cleanup the legacy code.
>
When I looked at patch 3, I was thinking the update follow_huge_pmd routine
would work for the PTE level with a few more modifications. Perhaps, this is
too ugly but it is a smaller set of changes for backport.
Of course, this would be followed up with the simplification patch which
removes all this code.
--
Mike Kravetz
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 852f911d676e..b2050d22d855 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -207,8 +207,8 @@ struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
struct page *follow_huge_pd(struct vm_area_struct *vma,
unsigned long address, hugepd_t hpd,
int flags, int pdshift);
-struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
- pmd_t *pmd, int flags);
+struct page *follow_huge_pmd_pte(struct vm_area_struct *vma, unsigned long address,
+ int flags);
struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
pud_t *pud, int flags);
struct page *follow_huge_pgd(struct mm_struct *mm, unsigned long address,
@@ -319,8 +319,8 @@ static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
return NULL;
}
-static inline struct page *follow_huge_pmd(struct mm_struct *mm,
- unsigned long address, pmd_t *pmd, int flags)
+static inline struct page *follow_huge_pmd_pte(struct vm_area_struct *vma,
+ unsigned long address, int flags)
{
return NULL;
}
diff --git a/mm/gup.c b/mm/gup.c
index 66d8619e02ad..fda980b436ed 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -530,6 +530,13 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
(FOLL_PIN | FOLL_GET)))
return ERR_PTR(-EINVAL);
+
+ if (is_vm_hugetlb_page(vma)) {
+ page = follow_huge_pmd_pte(vma, address, flags);
+ if (page)
+ return page;
+ return no_page_table(vma, flags);
+ }
retry:
if (unlikely(pmd_bad(*pmd)))
return no_page_table(vma, flags);
@@ -662,7 +669,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
if (pmd_none(pmdval))
return no_page_table(vma, flags);
if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
- page = follow_huge_pmd(mm, address, pmd, flags);
+ page = follow_huge_pmd_pte(vma, address, flags);
if (page)
return page;
return no_page_table(vma, flags);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d0617d64d718..e2e54dc27b00 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7155,13 +7155,23 @@ follow_huge_pd(struct vm_area_struct *vma,
return NULL;
}
+/*
+ * Temporarily handles both PMDs and PTEs.
+ * How can there be hugetlb entries at the PTE level? One such example is
+ * CONT_PTE on arm64.
+ *
+ * The hack of handling both PMDs and PTEs is made for a stable backports.
+ * A cleanup and removal of this code is made upstream.
+ */
struct page * __weak
-follow_huge_pmd(struct mm_struct *mm, unsigned long address,
- pmd_t *pmd, int flags)
+follow_huge_pmd_pte(struct vm_area_struct *vma, unsigned long address,
+ int flags)
{
+ struct hstate *h = hstate_vma(vma);
+ struct mm_struct *mm = vma->vm_mm;
struct page *page = NULL;
spinlock_t *ptl;
- pte_t pte;
+ pte_t *ptep, pte;
/*
* FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
@@ -7171,17 +7181,15 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
return NULL;
retry:
- ptl = pmd_lockptr(mm, pmd);
- spin_lock(ptl);
- /*
- * make sure that the address range covered by this pmd is not
- * unmapped from other threads.
- */
- if (!pmd_huge(*pmd))
+ ptep = huge_pte_offset(mm, address, huge_page_size(h));
+ if (!ptep)
goto out;
- pte = huge_ptep_get((pte_t *)pmd);
+
+ ptl = huge_pte_lock(h, mm, ptep);
+ pte = huge_ptep_get(ptep);
if (pte_present(pte)) {
- page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
+ page = pte_page(pte) +
+ ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
/*
* try_grab_page() should always succeed here, because: a) we
* hold the pmd (ptl) lock, and b) we've just checked that the
@@ -7197,7 +7205,7 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
} else {
if (is_hugetlb_entry_migration(pte)) {
spin_unlock(ptl);
- __migration_entry_wait_huge((pte_t *)pmd, ptl);
+ __migration_entry_wait_huge(ptep, ptl);
goto retry;
}
/*
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