[PATCH v1] hugetlb: support FOLL_FORCE|FOLL_WRITE

Guillaume Morin
2024-12-04T18:36:50Z

FOLL_FORCE|FOLL_WRITE has never been properly supported for hugetlb
mappings. Since 1d8d14641fd94, we explicitly reject it. However
running software on hugetlb mappings is a useful optimization.
Multiple tools allow to use that such as Intel iodlr or
libhugetlbfs.

Cc: Muchun Song <muchun.song@linux.dev>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Eric Hagberg <ehagberg@janestreet.com>
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
---
mm/gup.c | 93 ++++++++++++++++++++++++++--------------------------
mm/hugetlb.c | 20 ++++++-----
2 files changed, 58 insertions(+), 55 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 746070a1d8bf..c680edf33248 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -587,6 +587,33 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
}
#endif /* CONFIG_HAVE_GUP_FAST */

+/* Common code for can_follow_write_* */
+static inline bool can_follow_write_common(struct page *page,
+ struct vm_area_struct *vma, unsigned int flags)
+{
+ /* Maybe FOLL_FORCE is set to override it? */
+ if (!(flags & FOLL_FORCE))
+ return false;
+
+ /* But FOLL_FORCE has no effect on shared mappings */
+ if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+ return false;
+
+ /* ... or read-only private ones */
+ if (!(vma->vm_flags & VM_MAYWRITE))
+ return false;
+
+ /* ... or already writable ones that just need to take a write fault */
+ if (vma->vm_flags & VM_WRITE)
+ return false;
+
+ /*
+ * See can_change_pte_writable(): we broke COW and could map the page
+ * writable if we have an exclusive anonymous page ...
+ */
+ return page && PageAnon(page) && PageAnonExclusive(page);
+}
+
static struct page *no_page_table(struct vm_area_struct *vma,
unsigned int flags, unsigned long address)
{
@@ -613,6 +640,22 @@ static struct page *no_page_table(struct vm_area_struct *vma,
}

#ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
+/* FOLL_FORCE can write to even unwritable PUDs in COW mappings. */
+static inline bool can_follow_write_pud(pud_t pud, struct page *page,
+ struct vm_area_struct *vma,
+ unsigned int flags)
+{
+ /* If the pud is writable, we can write to the page. */
+ if (pud_write(pud))
+ return true;
+
+ if (!can_follow_write_common(page, vma, flags))
+ return false;
+
+ /* ... and a write-fault isn't required for other reasons. */
+ return !vma_soft_dirty_enabled(vma) || pud_soft_dirty(pud);
+}
+
static struct page *follow_huge_pud(struct vm_area_struct *vma,
unsigned long addr, pud_t *pudp,
int flags, struct follow_page_context *ctx)
@@ -625,7 +668,8 @@ static struct page *follow_huge_pud(struct vm_area_struct *vma,

assert_spin_locked(pud_lockptr(mm, pudp));

- if ((flags & FOLL_WRITE) && !pud_write(pud))
+ if ((flags & FOLL_WRITE) &&
+ !can_follow_write_pud(pud, page, vma, flags))
return NULL;

if (!pud_present(pud))
@@ -677,27 +721,7 @@ static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
if (pmd_write(pmd))
return true;

- /* Maybe FOLL_FORCE is set to override it? */
- if (!(flags & FOLL_FORCE))
- return false;
-
- /* But FOLL_FORCE has no effect on shared mappings */
- if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
- return false;
-
- /* ... or read-only private ones */
- if (!(vma->vm_flags & VM_MAYWRITE))
- return false;
-
- /* ... or already writable ones that just need to take a write fault */
- if (vma->vm_flags & VM_WRITE)
- return false;
-
- /*
- * See can_change_pte_writable(): we broke COW and could map the page
- * writable if we have an exclusive anonymous page ...
- */
- if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+ if (!can_follow_write_common(page, vma, flags))
return false;

/* ... and a write-fault isn't required for other reasons. */
@@ -798,27 +822,7 @@ static inline bool can_follow_write_pte(pte_t pte, struct page *page,
if (pte_write(pte))
return true;

- /* Maybe FOLL_FORCE is set to override it? */
- if (!(flags & FOLL_FORCE))
- return false;
-
- /* But FOLL_FORCE has no effect on shared mappings */
- if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
- return false;
-
- /* ... or read-only private ones */
- if (!(vma->vm_flags & VM_MAYWRITE))
- return false;
-
- /* ... or already writable ones that just need to take a write fault */
- if (vma->vm_flags & VM_WRITE)
- return false;
-
- /*
- * See can_change_pte_writable(): we broke COW and could map the page
- * writable if we have an exclusive anonymous page ...
- */
- if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+ if (!can_follow_write_common(page, vma, flags))
return false;

/* ... and a write-fault isn't required for other reasons. */
@@ -1285,9 +1289,6 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
if (!(gup_flags & FOLL_FORCE))
return -EFAULT;
- /* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
- if (is_vm_hugetlb_page(vma))
- return -EFAULT;
/*
* We used to let the write,force case do COW in a
* VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ea2ed8e301ef..52517b7ce308 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5169,6 +5169,13 @@ static void set_huge_ptep_writable(struct vm_area_struct *vma,
update_mmu_cache(vma, address, ptep);
}

+static void set_huge_ptep_maybe_writable(struct vm_area_struct *vma,
+ unsigned long address, pte_t *ptep)
+{
+ if (vma->vm_flags & VM_WRITE)
+ set_huge_ptep_writable(vma, address, ptep);
+}
+
bool is_hugetlb_entry_migration(pte_t pte)
{
swp_entry_t swp;
@@ -5802,13 +5809,6 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
if (!unshare && huge_pte_uffd_wp(pte))
return 0;

- /*
- * hugetlb does not support FOLL_FORCE-style write faults that keep the
- * PTE mapped R/O such as maybe_mkwrite() would do.
- */
- if (WARN_ON_ONCE(!unshare && !(vma->vm_flags & VM_WRITE)))
- return VM_FAULT_SIGSEGV;
-
/* Let's take out MAP_SHARED mappings first. */
if (vma->vm_flags & VM_MAYSHARE) {
set_huge_ptep_writable(vma, vmf->address, vmf->pte);
@@ -5837,7 +5837,8 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
SetPageAnonExclusive(&old_folio->page);
}
if (likely(!unshare))
- set_huge_ptep_writable(vma, vmf->address, vmf->pte);
+ set_huge_ptep_maybe_writable(vma, vmf->address,
+ vmf->pte);

delayacct_wpcopy_end();
return 0;
@@ -5943,7 +5944,8 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
spin_lock(vmf->ptl);
vmf->pte = hugetlb_walk(vma, vmf->address, huge_page_size(h));
if (likely(vmf->pte && pte_same(huge_ptep_get(mm, vmf->address, vmf->pte), pte))) {
- pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare);
+ const bool writable = !unshare && (vma->vm_flags & VM_WRITE);
+ pte_t newpte = make_huge_pte(vma, &new_folio->page, writable);

/* Break COW or unshare */
huge_ptep_clear_flush(vma, vmf->address, vmf->pte);
--
2.39.1

--
Guillaume Morin <guillaume@morinfr.org>