Re: [PATCH v2 2/2] mm: Allocate THP on hugezeropage wp-fault
From: Dev Jain
Date: Fri Sep 06 2024 - 03:05:59 EST
On 9/5/24 18:44, Ryan Roberts wrote:
On 04/09/2024 11:09, Dev Jain wrote:
Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
replace it with a PMD-mapped THP. Change the helpers introduced in the
previous patch to flush TLB entry corresponding to the hugezeropage,
and preserve PMD uffd-wp marker. In case of failure, fallback to
splitting the PMD.
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
include/linux/huge_mm.h | 6 ++++
mm/huge_memory.c | 79 +++++++++++++++++++++++++++++++++++------
mm/memory.c | 5 +--
3 files changed, 78 insertions(+), 12 deletions(-)
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index e25d9ebfdf89..fdd2cf473a3c 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -9,6 +9,12 @@
#include <linux/kobject.h>
vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
+vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
+ unsigned long haddr, struct folio **foliop,
+ unsigned long addr);
+void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+ struct vm_area_struct *vma, unsigned long haddr,
+ pgtable_t pgtable);
I don't think you are using either of these outside of huge_memory.c, so not
sure you need to declare them here or make them non-static?
As pointed out by Kirill, you are right, I forgot to drop these from my previous
approach.
int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 58125fbcc532..150163ad77d3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -943,9 +943,9 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
}
EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
-static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
- unsigned long haddr, struct folio **foliop,
- unsigned long addr)
+vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
+ unsigned long haddr, struct folio **foliop,
+ unsigned long addr)
{
struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
@@ -984,21 +984,29 @@ static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
}
-static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
- struct vm_area_struct *vma, unsigned long haddr,
- pgtable_t pgtable)
+void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+ struct vm_area_struct *vma, unsigned long haddr,
+ pgtable_t pgtable)
{
- pmd_t entry;
+ pmd_t entry, old_pmd;
+ bool is_pmd_none = pmd_none(*vmf->pmd);
entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
folio_add_lru_vma(folio, vma);
- pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
+ if (!is_pmd_none) {
+ old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
+ if (pmd_uffd_wp(old_pmd))
+ entry = pmd_mkuffd_wp(entry);
I don't really get this; entry is writable, so I wouldn't expect to also be
setting uffd-wp here? That combination is not allowed and is checked for in
page_table_check_pte_flags().
It looks like you expect to get here in the uffd-wp-async case, which used to
cause the pmd to be split to ptes. I'm guessing (but don't know for sure) that
would cause the uffd-wp bit to be set in each of the new ptes, then during
fallback to handling the wp fault on the pte, uffd would handle it?
I guess you are correct; I missed the WARN_ON() in page_table_check_pmd_flags(),
but I did see, if I read the uffd code correctly, that mfill_atomic() will just
return in case of pmd_trans_huge(*dst_pmd) while doing a uffd_copy to the destination
location. So preserving pmd_uffd_wp is useless in case a THP is mapped, but I did not
know that in fact it is supposed to be an invalid combination. So, I will drop it,
unless someone has some other objection.
+ }
+ if (pgtable)
+ pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
Should this call be moved outside of here? It doesn't really feel like it
belongs. Could it be called before calling map_pmd_thp() for the site that has a
pgtable?
Every other place I checked, they are doing this: make the entry -> deposit pgtable ->
set_pmd_at(). I guess the general flow is to do the deposit based on the old pmd, before
setting the new one. Which implies: I should at least move this check before I call
pmdp_huge_clear_flush(). And, since vmf->pmd and creating the new entry has no relation,
I am inclined to do what you are saying.
set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
- mm_inc_nr_ptes(vma->vm_mm);
+ if (is_pmd_none)
+ mm_inc_nr_ptes(vma->vm_mm);
}
static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
@@ -1576,6 +1584,50 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
spin_unlock(vmf->ptl);
}
+static vm_fault_t do_huge_zero_wp_pmd_locked(struct vm_fault *vmf,
+ unsigned long haddr,
+ struct folio *folio)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ vm_fault_t ret = 0;
+
+ ret = check_stable_address_space(vma->vm_mm);
+ if (ret)
+ goto out;
+ map_pmd_thp(folio, vmf, vma, haddr, NULL);
+out:
+ return ret;
+}
+
+static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long haddr)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ gfp_t gfp = vma_thp_gfp_mask(vma);
+ struct mmu_notifier_range range;
+ struct folio *folio = NULL;
+ vm_fault_t ret = 0;
+
+ ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
+ vmf->address);
Just checking: the PTE table was already allocated during the read fault, right?
So we don't have to allocate it here.
Correct, that happens in set_huge_zero_folio(). Thanks for checking.
+ if (ret)
+ goto out;
+
+ mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
+ haddr + HPAGE_PMD_SIZE);
+ mmu_notifier_invalidate_range_start(&range);
+ vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+ if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
+ goto unlock;
+ ret = do_huge_zero_wp_pmd_locked(vmf, haddr, folio);
+ if (!ret)
+ __thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
+unlock:
+ spin_unlock(vmf->ptl);
+ mmu_notifier_invalidate_range_end(&range);
I'll confess I don't understand all the mmu notifier rules.
I confess the same :)
But the doc at
Documentation/mm/mmu_notifier.rst implies that the notification must be done
while holding the PTL. Although that's not how wp_page_copy(). Are you confident
what you have done is correct?
Everywhere else, invalidate_range_end() is getting called after dropping the lock,
one reason is that it has a might_sleep(), and therefore we cannot call it while
holding a spinlock. I still don't know what exactly these calls mean...but I think
what I have done is correct.
Thanks,
Ryan
+out:
+ return ret;
+}
+
vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
{
const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
@@ -1588,8 +1640,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
VM_BUG_ON_VMA(!vma->anon_vma, vma);
- if (is_huge_zero_pmd(orig_pmd))
+ if (is_huge_zero_pmd(orig_pmd)) {
+ vm_fault_t ret = do_huge_zero_wp_pmd(vmf, haddr);
+
+ if (!(ret & VM_FAULT_FALLBACK))
+ return ret;
+
+ /* Fallback to splitting PMD if THP cannot be allocated */
goto fallback;
+ }
spin_lock(vmf->ptl);
diff --git a/mm/memory.c b/mm/memory.c
index 3c01d68065be..c081a25f5173 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5409,9 +5409,10 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
if (vma_is_anonymous(vma)) {
if (likely(!unshare) &&
userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) {
- if (userfaultfd_wp_async(vmf->vma))
+ if (!userfaultfd_wp_async(vmf->vma))
+ return handle_userfault(vmf, VM_UFFD_WP);
+ if (!is_huge_zero_pmd(vmf->orig_pmd))
goto split;
- return handle_userfault(vmf, VM_UFFD_WP);
}
return do_huge_pmd_wp_page(vmf);
}