[PATCH v1] hugetlb: support FOLL_FORCE|FOLL_WRITE

[Guillaume Morin <guillaume@morinfr.org>]

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>