The patch titled
Subject: mm: add functions folio_in_range() and folio_within_vma()
has been added to the -mm mm-unstable branch. Its filename is
mm-add-functions-folio_in_range-and-folio_within_vma.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-add-functions-folio_in_range-and-folio_within_vma.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Subject: mm: add functions folio_in_range() and folio_within_vma()
Date: Mon, 18 Sep 2023 15:33:16 +0800
Patch series "support large folio for mlock", v3.
Yu mentioned at [1] about the mlock() can't be applied to large folio.
I leant the related code and here is my understanding:
- For RLIMIT_MEMLOCK related, there is no problem. Because the
RLIMIT_MEMLOCK statistics is not related underneath page. That means
underneath page mlock or munlock doesn't impact the RLIMIT_MEMLOCK
statistics collection which is always correct.
- For keeping the page in RAM, there is no problem either. At least,
during try_to_unmap_one(), once detect the VMA has VM_LOCKED bit set in
vm_flags, the folio will be kept whatever the folio is mlocked or not.
So the function of mlock for large folio works. But it's not optimized
because the page reclaim needs scan these large folio and may split them.
This series identified the large folio for mlock to four types:
- The large folio is in VM_LOCKED range and fully mapped to the
range
- The large folio is in the VM_LOCKED range but not fully mapped to
the range
- The large folio cross VM_LOCKED VMA boundary
- The large folio cross last level page table boundary
For the first type, we mlock large folio so page reclaim will skip it.
For the second/third type, we don't mlock large folio. As the pages not
mapped to VM_LOACKED range are mapped to none VM_LOCKED range, if system
is in memory pressure situation, the large folio can be picked by page
reclaim and split. Then the pages not mapped to VM_LOCKED range can be
reclaimed.
For the fourth type, we don't mlock large folio because locking one page
table lock can't prevent the part in another last level page table being
unmapped. Thanks to Ryan for pointing this out.
To check whether the folio is fully mapped to the range, PTEs needs be
checked to see whether the page of folio is associated. Which needs take
page table lock and is heavy operation. So far, the only place needs this
check is madvise and page reclaim. These functions already have their own
PTE iterator.
patch1 introduce API to check whether large folio is in VMA range.
patch2 make page reclaim/mlock_vma_folio/munlock_vma_folio support
large folio mlock/munlock.
patch3 make mlock/munlock syscall support large folio.
Yu also mentioned a race which can make folio unevictable after munlock
during RFC v2 discussion [3]:
We decided that race issue didn't block this series based on:
- That race issue was not introduced by this series
- We had a looks-ok fix for that race issue. Need to wait
for mlock_count fixing patch as Yosry Ahmed suggested [4]
[1] https://lore.kernel.org/linux-mm/CAOUHufbtNPkdktjt_5qM45GegVO-rCFOMkSh0HQminQ12zsV8Q@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20230809061105.3369958-1-fengwei.yin@xxxxxxxxx/
[3] https://lore.kernel.org/linux-mm/CAOUHufZ6=9P_=CAOQyw0xw-3q707q-1FVV09dBNDC-hpcpj2Pg@xxxxxxxxxxxxxx/
This patch (of 3):
folio_in_range() will be used to check whether the folio is mapped to
specific VMA and whether the mapping address of folio is in the range.
Also a helper function folio_within_vma() to check whether folio
is in the range of vma based on folio_in_range().
Link: https://lkml.kernel.org/r/20230918073318.1181104-1-fengwei.yin@xxxxxxxxx
Link: https://lkml.kernel.org/r/20230918073318.1181104-2-fengwei.yin@xxxxxxxxx
Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/internal.h | 50 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)
--- a/mm/internal.h~mm-add-functions-folio_in_range-and-folio_within_vma
+++ a/mm/internal.h
@@ -587,6 +587,56 @@ extern long faultin_vma_page_range(struc
bool write, int *locked);
extern bool mlock_future_ok(struct mm_struct *mm, unsigned long flags,
unsigned long bytes);
+
+/*
+ * NOTE: This function can't tell whether the folio is "fully mapped" in the
+ * range.
+ * "fully mapped" means all the pages of folio is associated with the page
+ * table of range while this function just check whether the folio range is
+ * within the range [start, end). Funcation caller nees to do page table
+ * check if it cares about the page table association.
+ *
+ * Typical usage (like mlock or madvise) is:
+ * Caller knows at least 1 page of folio is associated with page table of VMA
+ * and the range [start, end) is intersect with the VMA range. Caller wants
+ * to know whether the folio is fully associated with the range. It calls
+ * this function to check whether the folio is in the range first. Then checks
+ * the page table to know whether the folio is fully mapped to the range.
+ */
+static inline bool
+folio_within_range(struct folio *folio, struct vm_area_struct *vma,
+ unsigned long start, unsigned long end)
+{
+ pgoff_t pgoff, addr;
+ unsigned long vma_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+
+ VM_WARN_ON_FOLIO(folio_test_ksm(folio), folio);
+ if (start > end)
+ return false;
+
+ if (start < vma->vm_start)
+ start = vma->vm_start;
+
+ if (end > vma->vm_end)
+ end = vma->vm_end;
+
+ pgoff = folio_pgoff(folio);
+
+ /* if folio start address is not in vma range */
+ if (!in_range(pgoff, vma->vm_pgoff, vma_pglen))
+ return false;
+
+ addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+
+ return !(addr < start || end - addr < folio_size(folio));
+}
+
+static inline bool
+folio_within_vma(struct folio *folio, struct vm_area_struct *vma)
+{
+ return folio_within_range(folio, vma, vma->vm_start, vma->vm_end);
+}
+
/*
* mlock_vma_folio() and munlock_vma_folio():
* should be called with vma's mmap_lock held for read or write,
_
Patches currently in -mm which might be from fengwei.yin@xxxxxxxxx are
filemap-add-filemap_map_order0_folio-to-handle-order0-folio.patch
mm-add-functions-folio_in_range-and-folio_within_vma.patch
mm-handle-large-folio-when-large-folio-in-vm_locked-vma-range.patch
mm-mlock-update-mlock_pte_range-to-handle-large-folio.patch
