On 1/20/2025 9:08 PM, Jiaqi Yan wrote:
On Mon, Jan 20, 2025 at 9:01 PM <jane.chu@xxxxxxxxxx> wrote:
On 1/20/2025 5:21 PM, Jiaqi Yan wrote:
On Mon, Jan 20, 2025 at 2:59 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 19.01.25 19:06, Jiaqi Yan wrote:
While I was working on userspace MFR via memfd [1], I spend some time to
understand what current kernel does when a HugeTLB-backing memfd is
truncated. My expectation is, if there is a HWPoison HugeTLB folio
mapped via the memfd to userspace, it will be unmapped right away but
still be kept in page cache [2]; however when the memfd is truncated to
zero or after the memfd is closed, kernel should dissolve the HWPoison
folio in the page cache, and free only the clean raw pages to buddy
allocator, excluding the poisoned raw page.
So I wrote a hugetlb-mfr-base.c selftest and expect
0. say nr_hugepages initially is 64 as system configuration.
1. after MADV_HWPOISON, nr_hugepages should still be 64 as we kept even
HWPoison huge folio in page cache. free_hugepages should be
nr_hugepages minus whatever the amount in use.
2. after truncated memfd to zero, nr_hugepages should reduced to 63 as
kernel dissolved and freed the HWPoison huge folio. free_hugepages
should also be 63.
However, when testing at the head of mm-stable commit 2877a83e4a0a
("mm/hugetlb: use folio->lru int demote_free_hugetlb_folios()"), I found
although free_hugepages is reduced to 63, nr_hugepages is not reduced
and stay at 64.
Is my expectation outdated? Or is this some kind of bug?
I assume this is a bug and then digged a little bit more. It seems there
are two issues, or two things I don't really understand.
1. During try_memory_failure_hugetlb, we should increased the target
in-use folio's refcount via get_hwpoison_hugetlb_folio. However,
until the end of try_memory_failure_hugetlb, this refcout is not put.
I can make sense of this given we keep in-use huge folio in page
cache.
Isn't the general rule that hwpoisoned folios have a raised refcount
such that they won't get freed + reused? At least that's how the buddy
deals with them, and I suspect also hugetlb?
Thanks, David.
I see, so it is expected that the _entire_ huge folio will always have
at least a refcount of 1, even when the folio can become "free".
For *free* huge folio, try_memory_failure_hugetlb dissolves it and
frees the clean pages (a lot) to the buddy allocator. This made me
think the same thing will happen for *in-use* huge folio _eventually_
(i.e. somehow the refcount due to HWPoison can be put). I feel this is
a little bit unfortunate for the clean pages, but if it is what it is,
that's fair as it is not a bug.
Agreed with David. For *in use* hugetlb pages, including unused shmget
pages, hugetlb shouldn't dissvolve the page, not until an explicit freeing action is taken like
RMID and echo 0 > nr_hugepages.
To clarify myself, I am not asking memory-failure.c to dissolve the
hugepage at the time it is in-use, but rather when it becomes free
(truncated or process exited).
Understood, a free hugetlb page in the pool should have refcount 1 though.
-jane
-jane
[ 1069.320976] page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2780000
[ 1069.320978] head: order:18 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
[ 1069.320980] flags: 0x400000000100044(referenced|head|hwpoison|node=0|zone=1)
[ 1069.320982] page_type: f4(hugetlb)
[ 1069.320984] raw: 0400000000100044 ffffffff8760bbc8 ffffffff8760bbc8 0000000000000000
[ 1069.320985] raw: 0000000000000000 0000000000000000 00000001f4000000 0000000000000000
[ 1069.320987] head: 0400000000100044 ffffffff8760bbc8 ffffffff8760bbc8 0000000000000000
[ 1069.320988] head: 0000000000000000 0000000000000000 00000001f4000000 0000000000000000
[ 1069.320990] head: 0400000000000012 ffffdd53de000001 ffffffffffffffff 0000000000000000
[ 1069.320991] head: 0000000000040000 0000000000000000 00000000ffffffff 0000000000000000
[ 1069.320992] page dumped because: track hwpoison folio's ref
2. Even if folio's refcount do drop to zero and we get into
free_huge_folio, it is not clear to me which part of free_huge_folio
is handling the case that folio is HWPoison. In my test what I
observed is that evantually the folio is enqueue_hugetlb_folio()-ed.
How would we get a refcount of 0 if we assume the raised refcount on a
hwpoisoned hugetlb folio?
I'm probably missing something: are you saying that you can trigger a
hwpoisoned hugetlb folio to get reallocated again, in upstream code?
No, I think it is just my misunderstanding. From what you said, the
expectation of HWPoison hugetlb folio is just it won't get reallocated
again, which is true.
My (wrong) expectation is, in addition to the "won't reallocated
again" part, some (large) portion of the huge folio will be freed to
the buddy allocator. On the other hand, is it something worth having /
improving? (1G - some_single_digit * 4KB) seems to be valuable to the
system, though they are all 4K. #1 and #2 above are then what needs to
be done if the improvement is worth chasing.
--
Cheers,
David / dhildenb
