Re: [PATCH mm-unstable v1 1/4] mm/swap: stop using page->private on tail pages for THP_SWAP
From: David Hildenbrand
Date: Wed Aug 23 2023 - 11:27:47 EST
On 23.08.23 17:21, Yosry Ahmed wrote:
On Wed, Aug 23, 2023 at 8:17 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 23.08.23 17:12, Yosry Ahmed wrote:
On Mon, Aug 21, 2023 at 9:09 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
Let's stop using page->private on tail pages, making it possible to
just unconditionally reuse that field in the tail pages of large folios.
The remaining usage of the private field for THP_SWAP is in the THP
splitting code (mm/huge_memory.c), that we'll handle separately later.
Update the THP_SWAP documentation and sanity checks in mm_types.h and
__split_huge_page_tail().
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
The mm part looks good to me (with the added fixup):
Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Thanks!
/**
diff --git a/include/linux/swap.h b/include/linux/swap.h
index bb5adc604144..84fe0e94f5cd 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -339,6 +339,15 @@ static inline swp_entry_t folio_swap_entry(struct folio *folio)
return entry;
}
+static inline swp_entry_t page_swap_entry(struct page *page)
+{
+ struct folio *folio = page_folio(page);
+ swp_entry_t entry = folio_swap_entry(folio);
+
+ entry.val += page - &folio->page;
+ return entry;
+}
+
static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
{
folio->private = (void *)entry.val;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index cc2f65f8cc62..c04702ae71d2 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2446,18 +2446,15 @@ static void __split_huge_page_tail(struct page *head, int tail,
page_tail->index = head->index + tail;
/*
- * page->private should not be set in tail pages with the exception
- * of swap cache pages that store the swp_entry_t in tail pages.
- * Fix up and warn once if private is unexpectedly set.
- *
- * What of 32-bit systems, on which folio->_pincount overlays
- * head[1].private? No problem: THP_SWAP is not enabled on 32-bit, and
- * pincount must be 0 for folio_ref_freeze() to have succeeded.
+ * page->private should not be set in tail pages. Fix up and warn once
+ * if private is unexpectedly set.
*/
- if (!folio_test_swapcache(page_folio(head))) {
- VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
+ if (unlikely(page_tail->private)) {
+ VM_WARN_ON_ONCE_PAGE(true, page_tail);
page_tail->private = 0;
}
Could probably save a couple of lines here:
if (VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail))
page_tail->private = 0;
That would mean that we eventually compile out the runtime check
#define VM_WARN_ON_ONCE_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
I thought the warning would be compiled out but not the check, my bad.
I even remembered that VM_WARN_ON_ONCE and friends could/should not be
used in conditionals.
But we do seem to have two users now:
$ git grep "if (VM_WARN_ON"
mm/mmap.c: if (VM_WARN_ON_ONCE_MM(vma->vm_end != vmi_end, mm))
mm/mmap.c: if (VM_WARN_ON_ONCE_MM(vma->vm_start != vmi_start, mm))
But they only do warning-related action, to dump the stack, the vma, ...
So if the warnings get compiled out, also all the other stuff gets compiled out as well,
which makes sense here.
--
Cheers,
David / dhildenb