Re: [PATCH v6 0/5] Only free healthy pages in high-order has_hwpoisoned folio

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/28/26 19:18, Vlastimil Babka (SUSE) wrote:
> On 7/27/26 16:20, David Hildenbrand (Arm) wrote:
>> On 7/22/26 10:27, Vlastimil Babka (SUSE) wrote:
>>>
>>> Would it truly fix the issue, or rather there would still be a race window
>>> left where we check that there's no hwpoison flag in the re-enabled check,
>>> and only then someone sets it?
>>
>> Why are we checking PageHWPoison at all then in check_new_page()?
> 
> We check all kinds of unexpected state, when that's enable. PageHWPoison
> could have been considered unexpected too, when the checks were made more
> and more optional (first by Mel and then me).
> 
> But indeed it seems the PageHWPoison check is supposed to be load-bearing
> (hi, Lorenzo!). It's intentionally handled before bad_page() (with a taint)
> in check_new_page_bad(). Commits 2a7684a23e9c and f4c18e6f7b5b are also a hint.
> 
>> I think we created a mess.
> 
> Yes, the PageHWPoison handling was made part of debugging sanity check and
> then not recognized properly as load-bearing later.

BTW, I'd assume we can check for PageHWPoison at free time fairly efficiently, as we
touch all page flags already.

Something along the lines of:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 520ebd2fa40b5..57f89f8684168 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1321,6 +1321,7 @@ static __always_inline bool __free_pages_prepare(struct page *page,
        bool init = want_init_on_free();
        bool compound = PageCompound(page);
        struct folio *folio = page_folio(page);
+       unsigned int hwpoisoned = 0;
 
        if (fpi_flags & FPI_PREPARED)
                return true;
@@ -1347,21 +1348,6 @@ static __always_inline bool __free_pages_prepare(struct page *page,
                count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
        }
 
-       if (unlikely(PageHWPoison(page)) && !order) {
-               /* Do not let hwpoison pages hit pcplists/buddy */
-               reset_page_owner(page, order);
-               page_table_check_free(page, order);
-               pgalloc_tag_sub(page, 1 << order);
-
-               /*
-                * The page is isolated and accounted for.
-                * Mark the codetag as empty to avoid accounting error
-                * when the page is freed by unpoison_memory().
-                */
-               clear_page_tag_ref(page);
-               return false;
-       }
-
        VM_BUG_ON_PAGE(compound && compound_order(page) != order, page);
 
        /*
@@ -1395,8 +1381,26 @@ static __always_inline bool __free_pages_prepare(struct page *page,
                                }
                        }
                        tail_page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
+                       hwpoisoned += PageHWPoison(tail_page);
                }
        }
+       hwpoisoned += PageHWPoison(page);
+
+       if (unlikely(hwpoisoned)) {
+               /* Do not let hwpoison pages hit pcplists/buddy */
+               reset_page_owner(page, order);
+               page_table_check_free(page, order);
+               pgalloc_tag_sub(page, 1 << order);
+
+               /*
+                * The page is isolated and accounted for.
+                * Mark the codetag as empty to avoid accounting error
+                * when the page is freed by unpoison_memory().
+                */
+               clear_page_tag_ref(page);
+               return false;
+       }
+
        if (folio_test_anon(folio)) {
                mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
                folio->mapping = NULL;
diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile


Of course, we could also fallback for order>0 to do some smart splitting.


Checking on allocation efficiently is indeed a bit more tricky. For compound
pages prep_compound_page() will already walk all pages.

Having hwpoison logic to just sync with the buddy when setting hwpoison flags
would be nicer.

Then we could (maybe) have the rule that no hwpoisoned page can enter the buddy, and no
page can become hwpoisoned while in the buddy. Consequently, no hwpoisoned page can
leave the buddy (except weird races while allocating, but then it's just an allocated
page).

Maybe.

-- 
Cheers,

David