Re: [syzbot] [mm?] WARNING in memory_failure
From: jane . chu
Date: Wed Oct 01 2025 - 14:17:00 EST
On 9/29/2025 11:31 PM, Miaohe Lin wrote:
On 2025/9/30 12:35, jane.chu@xxxxxxxxxx wrote:
On 9/29/2025 7:51 PM, Miaohe Lin wrote:
On 2025/9/30 2:23, jane.chu@xxxxxxxxxx wrote:
On 9/29/2025 10:49 AM, jane.chu@xxxxxxxxxx wrote:
On 9/29/2025 10:29 AM, jane.chu@xxxxxxxxxx wrote:
On 9/29/2025 4:08 AM, Pankaj Raghav (Samsung) wrote:
I want to change all the split functions in huge_mm.h and provide
mapping_min_folio_order() to try_folio_split() in truncate_inode_partial_folio().
Something like below:
1. no split function will change the given order;
2. __folio_split() will no longer give VM_WARN_ONCE when provided new_order
is smaller than mapping_min_folio_order().
In this way, for an LBS folio that cannot be split to order 0, split
functions will return -EINVAL to tell caller that the folio cannot
be split. The caller is supposed to handle the split failure.
IIUC, we will remove warn on once but just return -EINVAL in __folio_split()
function if new_order < min_order like this:
...
min_order = mapping_min_folio_order(folio->mapping);
if (new_order < min_order) {
- VM_WARN_ONCE(1, "Cannot split mapped folio below min- order: %u",
- min_order);
ret = -EINVAL;
goto out;
}
...
Then the user process will get a SIGBUS indicting the entire huge page at higher order -
folio_set_has_hwpoisoned(folio);
if (try_to_split_thp_page(p, false) < 0) {
res = -EHWPOISON;
kill_procs_now(p, pfn, flags, folio);
put_page(p);
action_result(pfn, MF_MSG_UNSPLIT_THP, MF_FAILED);
goto unlock_mutex;
}
VM_BUG_ON_PAGE(!page_count(p), p);
folio = page_folio(p);
the huge page is not usable any way, kind of similar to the hugetlb page situation: since the page cannot be splitted, the entire page is marked unusable.
How about keep the current huge page split code as is, but change the M- F code to recognize that in a successful splitting case, the poisoned page might just be in a lower folio order, and thus, deliver the SIGBUS ?
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index a24806bb8e82..342c81edcdd9 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2291,7 +2291,9 @@ int memory_failure(unsigned long pfn, int flags)
* page is a valid handlable page.
*/
folio_set_has_hwpoisoned(folio);
- if (try_to_split_thp_page(p, false) < 0) {
+ ret = try_to_split_thp_page(p, false);
+ folio = page_folio(p);
+ if (ret < 0 || folio_test_large(folio)) {
res = -EHWPOISON;
kill_procs_now(p, pfn, flags, folio);
put_page(p);
@@ -2299,7 +2301,6 @@ int memory_failure(unsigned long pfn, int flags)
goto unlock_mutex;
}
VM_BUG_ON_PAGE(!page_count(p), p);
- folio = page_folio(p);
}
thanks,
-jane
Maybe this is better, in case there are other reason for split_huge_page() to return -EINVAL.
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index a24806bb8e82..2bfa05acae65 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1659,9 +1659,10 @@ static int identify_page_state(unsigned long pfn, struct page *p,
static int try_to_split_thp_page(struct page *page, bool release)
{
int ret;
+ int new_order = min_order_for_split(page_folio(page));
lock_page(page);
- ret = split_huge_page(page);
+ ret = split_huge_page_to_list_to_order(page, NULL, new_order);
unlock_page(page);
if (ret && release)
@@ -2277,6 +2278,7 @@ int memory_failure(unsigned long pfn, int flags)
folio_unlock(folio);
if (folio_test_large(folio)) {
+ int ret;
/*
* The flag must be set after the refcount is bumped
* otherwise it may race with THP split.
@@ -2291,7 +2293,9 @@ int memory_failure(unsigned long pfn, int flags)
* page is a valid handlable page.
*/
folio_set_has_hwpoisoned(folio);
- if (try_to_split_thp_page(p, false) < 0) {
+ ret = try_to_split_thp_page(p, false);
+ folio = page_folio(p);
+ if (ret < 0 || folio_test_large(folio)) {
res = -EHWPOISON;
kill_procs_now(p, pfn, flags, folio);
put_page(p);
@@ -2299,7 +2303,6 @@ int memory_failure(unsigned long pfn, int flags)
goto unlock_mutex;
}
VM_BUG_ON_PAGE(!page_count(p), p);
- folio = page_folio(p);
}
/*
@@ -2618,7 +2621,8 @@ static int soft_offline_in_use_page(struct page *page)
};
if (!huge && folio_test_large(folio)) {
- if (try_to_split_thp_page(page, true)) {
+ if ((try_to_split_thp_page(page, true)) ||
+ folio_test_large(page_folio(page))) {
pr_info("%#lx: thp split failed\n", pfn);
return -EBUSY;
}
In soft offline, better to check if (min_order_for_split > 0), no need to split, just return for now ...
I might be miss something but why we have to split it? Could we migrate the whole thp or folio with min_order instead?
The soft offline code was originally written with the assumption that only 1 base page will be offlined.
Yes, only page corresponding to parameter @pfn of soft_offline_page() will be offlined.
With the recent introduction of min_order, it might quietly offline multiple pages, is that a desirable thing?
I don't think so. Even if try_to_split_thp_page splits folio into smaller one with min_order, page_handle_poison()
will put back the folio into buddy after migrate_pages, set the hwpoisoned flag to raw error page and hold the extra
refcnt. So only raw error page will be offlined while other sub-pages will be put back into buddy.
Or am I miss something?
The thing is that the non-poisoned subpages are also migrated away,
they're no longer available for the user process.
thanks,
-jane
Thanks.
.