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SubjectRe: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings
On 08/05/22 14:12, Peter Xu wrote:
> On Fri, Aug 05, 2022 at 01:03:29PM +0200, David Hildenbrand wrote:
> > Let's add a safety net if we ever get (again) a write-fault on a R/O-mapped
> > page in a shared mapping, in which case we simply have to map the
> > page writable.
> >

> > + if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
> > + return VM_FAULT_SIGSEGV;
>
> I had a feeling that you just want to double check we have write
> permission, but IIUC this should be checked far earlier or we'll have
> problem.

Back when I was exploring hugetlb softdirty support, this patch handled
this condition by not calling into hugetlb_wp (was hugetlb_cow).

https://lore.kernel.org/linux-mm/20210211000322.159437-3-mike.kravetz@oracle.com/

Here is a quickly updated version that was only tested with David's program.
--
Mike Kravetz

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a23f1cffaf49..86d38d41fddf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5758,9 +5758,10 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
* spinlock. For private mappings, we also lookup the pagecache
* page now as it is used to determine if a reservation has been
* consumed.
+ * Only private/non-shared mappings are sent to hugetlb_wp.
*/
if ((flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) &&
- !huge_pte_write(entry)) {
+ !huge_pte_write(entry) && !(vma->vm_flags & VM_MAYSHARE)) {
if (vma_needs_reservation(h, vma, haddr) < 0) {
ret = VM_FAULT_OOM;
goto out_mutex;
@@ -5768,8 +5769,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
/* Just decrements count, does not deallocate */
vma_end_reservation(h, vma, haddr);

- if (!(vma->vm_flags & VM_MAYSHARE))
- pagecache_page = hugetlbfs_pagecache_page(h,
+ pagecache_page = hugetlbfs_pagecache_page(h,
vma, haddr);
}

@@ -5815,9 +5815,14 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,

if (flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) {
if (!huge_pte_write(entry)) {
- ret = hugetlb_wp(mm, vma, address, ptep, flags,
+ if (!(vma->vm_flags & VM_MAYSHARE)) {
+ ret = hugetlb_wp(mm, vma, address, ptep, flags,
pagecache_page, ptl);
- goto out_put_page;
+ goto out_put_page;
+ }
+
+ entry = huge_pte_mkwrite(entry);
+ entry = huge_pte_mkdirty(entry);
} else if (likely(flags & FAULT_FLAG_WRITE)) {
entry = huge_pte_mkdirty(entry);
}
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