Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

From: Yang Shi
Date: Mon Jul 02 2018 - 13:20:02 EST



On 7/2/18 5:33 AM, Kirill A. Shutemov wrote:
On Sat, Jun 30, 2018 at 06:39:44AM +0800, Yang Shi wrote:
When running some mmap/munmap scalability tests with large memory (i.e.
300GB), the below hung task issue may happen occasionally.
INFO: task ps:14018 blocked for more than 120 seconds.
Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
ps D 0 14018 1 0x00000004
ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
Call Trace:
[<ffffffff817154d0>] ? __schedule+0x250/0x730
[<ffffffff817159e6>] schedule+0x36/0x80
[<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
[<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
[<ffffffff81717db0>] down_read+0x20/0x40
[<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
[<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
[<ffffffff81241d87>] __vfs_read+0x37/0x150
[<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
[<ffffffff81242266>] vfs_read+0x96/0x130
[<ffffffff812437b5>] SyS_read+0x55/0xc0
[<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5

It is because munmap holds mmap_sem from very beginning to all the way
down to the end, and doesn't release it in the middle. When unmapping
large mapping, it may take long time (take ~18 seconds to unmap 320GB
mapping with every single page mapped on an idle machine).

It is because munmap holds mmap_sem from very beginning to all the way
down to the end, and doesn't release it in the middle. When unmapping
large mapping, it may take long time (take ~18 seconds to unmap 320GB
mapping with every single page mapped on an idle machine).

Zapping pages is the most time consuming part, according to the
suggestion from Michal Hock [1], zapping pages can be done with holding
read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write
mmap_sem to cleanup vmas. All zapped vmas will have VM_DEAD flag set,
the page fault to VM_DEAD vma will trigger SIGSEGV.

Define large mapping size thresh as PUD size or 1GB, just zap pages with
read mmap_sem for mappings which are >= thresh value.

If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, then just
fallback to regular path since unmapping those mappings need acquire
write mmap_sem.

For the time being, just do this in munmap syscall path. Other
vm_munmap() or do_munmap() call sites remain intact for stability
reason.

The below is some regression and performance data collected on a machine
with 32 cores of E5-2680 @ 2.70GHz and 384GB memory.

With the patched kernel, write mmap_sem hold time is dropped to us level
from second.

[1] https://lwn.net/Articles/753269/

Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 136 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 134 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 87dcf83..d61e08b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2763,6 +2763,128 @@ static int munmap_lookup_vma(struct mm_struct *mm, struct vm_area_struct **vma,
return 1;
}
+/* Consider PUD size or 1GB mapping as large mapping */
+#ifdef HPAGE_PUD_SIZE
+#define LARGE_MAP_THRESH HPAGE_PUD_SIZE
+#else
+#define LARGE_MAP_THRESH (1 * 1024 * 1024 * 1024)
+#endif
PUD_SIZE is defined everywhere.

If THP is defined, otherwise it is:

#define HPAGE_PUD_SIZE ({ BUILD_BUG(); 0; })


+
+/* Unmap large mapping early with acquiring read mmap_sem */
+static int do_munmap_zap_early(struct mm_struct *mm, unsigned long start,
+ size_t len, struct list_head *uf)
+{
+ unsigned long end = 0;
+ struct vm_area_struct *vma = NULL, *prev, *tmp;
+ bool success = false;
+ int ret = 0;
+
+ if (!munmap_addr_sanity(start, len))
+ return -EINVAL;
+
+ len = PAGE_ALIGN(len);
+
+ end = start + len;
+
+ /* Just deal with uf in regular path */
+ if (unlikely(uf))
+ goto regular_path;
+
+ if (len >= LARGE_MAP_THRESH) {
+ /*
+ * need write mmap_sem to split vma and set VM_DEAD flag
+ * splitting vma up-front to save PITA to clean if it is failed
What errors do you talk about? ENOMEM on VMA split? Anything else?

Yes, ENOMEM on vma split.


+ */
+ down_write(&mm->mmap_sem);
+ ret = munmap_lookup_vma(mm, &vma, &prev, start, end);
+ if (ret != 1) {
+ up_write(&mm->mmap_sem);
+ return ret;
+ }
+ /* This ret value might be returned, so reset it */
+ ret = 0;
+
+ /*
+ * Unmapping vmas, which has VM_LOCKED|VM_HUGETLB|VM_PFNMAP
+ * flag set or has uprobes set, need acquire write map_sem,
+ * so skip them in early zap. Just deal with such mapping in
+ * regular path.
+ * Borrow can_madv_dontneed_vma() to check the conditions.
+ */
+ tmp = vma;
+ while (tmp && tmp->vm_start < end) {
+ if (!can_madv_dontneed_vma(tmp) ||
+ vma_has_uprobes(tmp, start, end)) {
+ up_write(&mm->mmap_sem);
+ goto regular_path;
+ }
+ tmp = tmp->vm_next;
+ }
+ /*
+ * set VM_DEAD flag before tear down them.
+ * page fault on VM_DEAD vma will trigger SIGSEGV.
+ */
+ tmp = vma;
+ for ( ; tmp && tmp->vm_start < end; tmp = tmp->vm_next)
+ tmp->vm_flags |= VM_DEAD;
I probably miss the explanation somewhere, but what's wrong with allowing
other thread to re-populate the VMA?

I would rather allow the VMA to be re-populated by other thread while we
are zapping the range. And later zap the range again under down_write.

It should also lead to consolidated regular path: take mmap_sem for write
and call do_munmap().

On the first path we just skip VMA we cannot deal with under
down_read(mmap_sem), regular path will take care of them.


+ up_write(&mm->mmap_sem);
+
+ /* zap mappings with read mmap_sem */
+ down_read(&mm->mmap_sem);
Yeah. There's race between up_write() and down_read().
Use downgrade, as Andrew suggested.

+ zap_page_range(vma, start, len);
+ /* indicates early zap is success */
+ success = true;
+ up_read(&mm->mmap_sem);
And here again.

This race can be avoided if we wouldn't carry vma to regular_path, but
just go directly to do_munmap().

Thanks, Kirill. Yes, I did think about re-validating vmas before. This sounds reasonable to avoid the race. Although we spend more time in re-looking up vmas, but it should be very short, and the duplicate zap should be very short too.

Yang


+ }
+
+regular_path:
+ /* hold write mmap_sem for vma manipulation or regular path */
+ if (down_write_killable(&mm->mmap_sem))
+ return -EINTR;
+ if (success) {
+ /* vmas have been zapped, here clean up pgtable and vmas */
+ struct vm_area_struct *next = prev ? prev->vm_next : mm->mmap;
+ struct mmu_gather tlb;
+ tlb_gather_mmu(&tlb, mm, start, end);
+ free_pgtables(&tlb, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
+ next ? next->vm_start : USER_PGTABLES_CEILING);
+ tlb_finish_mmu(&tlb, start, end);
+
+ detach_vmas_to_be_unmapped(mm, vma, prev, end);
+ arch_unmap(mm, vma, start, end);
+ remove_vma_list(mm, vma);
+ } else {
+ /* vma is VM_LOCKED|VM_HUGETLB|VM_PFNMAP or has uprobe */
+ if (vma) {
+ if (unlikely(uf)) {
+ int ret = userfaultfd_unmap_prep(vma, start,
+ end, uf);
+ if (ret)
+ goto out;
+ }
+ if (mm->locked_vm) {
+ tmp = vma;
+ while (tmp && tmp->vm_start < end) {
+ if (tmp->vm_flags & VM_LOCKED) {
+ mm->locked_vm -= vma_pages(tmp);
+ munlock_vma_pages_all(tmp);
+ }
+ tmp = tmp->vm_next;
+ }
+ }
+ detach_vmas_to_be_unmapped(mm, vma, prev, end);
+ unmap_region(mm, vma, prev, start, end);
+ remove_vma_list(mm, vma);
+ } else
+ /* When mapping size < LARGE_MAP_THRESH */
+ ret = do_munmap(mm, start, len, uf);
+ }
+
+out:
+ up_write(&mm->mmap_sem);
+ return ret;
+}
+
/* Munmap is split into 2 main parts -- this part which finds
* what needs doing, and the areas themselves, which do the
* work. This now handles partial unmappings.
@@ -2829,6 +2951,17 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
return 0;
}
+static int vm_munmap_zap_early(unsigned long start, size_t len)
+{
+ int ret;
+ struct mm_struct *mm = current->mm;
+ LIST_HEAD(uf);
+
+ ret = do_munmap_zap_early(mm, start, len, &uf);
+ userfaultfd_unmap_complete(mm, &uf);
+ return ret;
+}
+
int vm_munmap(unsigned long start, size_t len)
{
int ret;
@@ -2848,10 +2981,9 @@ int vm_munmap(unsigned long start, size_t len)
SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
{
profile_munmap(addr);
- return vm_munmap(addr, len);
+ return vm_munmap_zap_early(addr, len);
}
-
/*
* Emulation of deprecated remap_file_pages() syscall.
*/
--
1.8.3.1