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Date: Wed, 16 Aug 2023 12:53:34 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
linux-mm@...ck.org, Hugh Dickins <hughd@...gle.com>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Randy Dunlap <rdunlap@...radead.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Yu Zhao <yuzhao@...gle.com>,
Ryan Roberts <ryan.roberts@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH RFC v3] mm: Proper document tail pages fields for folio
On Wed, Aug 16, 2023 at 03:33:30PM +0200, David Hildenbrand wrote:
> On 15.08.23 23:25, Peter Xu wrote:
> > Tail page struct reuse is over-comlicated. Not only because we have
>
> It is complicated, agreed.
>
> With the ->private for THP_SWAP gone, we would have to document less.
> Stating that 4*4byte / 4*8 byte are available after flags+head would
> be sufficient and I'd even drop the table.
>
>
> > implicit uses of tail page fields (mapcounts, or private for thp swap
> > support, etc., that we may still use in the page structs,
>
> Instead of documenting that thp swap should no longer touch the private
> field of tail pages, maybe we can indeed fix that quite easily.
>
> My simple tests passed so far. If there isn't something obvious missing,
> I can do more testing and send this as an official patch.
It'll be definitely good to fix it rather than document if possible.
Nothing wrong I spot quickly, you may just need a more complete cc list for
swap. One trivial comment below.
>
>
> From ec0f8b0dd8fb81c316b6a4c5fc9ae7563e625404 Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@...hat.com>
> Date: Wed, 16 Aug 2023 13:14:45 +0200
> Subject: [PATCH] mm/swap: stop using page->private on tail pages for THP_SWAP
>
> 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.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> arch/arm64/mm/mteswap.c | 5 +++--
> include/linux/swap.h | 9 +++++++++
> mm/huge_memory.c | 15 ++++++---------
> mm/memory.c | 2 +-
> mm/rmap.c | 2 +-
> mm/swap_state.c | 4 ++--
> mm/swapfile.c | 4 ++--
> 7 files changed, 24 insertions(+), 17 deletions(-)
>
> diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
> index cd508ba80ab1..a31833e3ddc5 100644
> --- a/arch/arm64/mm/mteswap.c
> +++ b/arch/arm64/mm/mteswap.c
> @@ -33,8 +33,9 @@ int mte_save_tags(struct page *page)
> mte_save_page_tags(page_address(page), tag_storage);
> - /* page_private contains the swap entry.val set in do_swap_page */
> - ret = xa_store(&mte_pages, page_private(page), tag_storage, GFP_KERNEL);
> + /* lookup the swap entry.val from the page */
> + ret = xa_store(&mte_pages, page_swap_entry(page).val, tag_storage,
> + GFP_KERNEL);
> if (WARN(xa_is_err(ret), "Failed to store MTE tags")) {
> mte_free_tag_storage(tag_storage);
> return xa_err(ret);
> 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 0b709d2c46c6..f7e04cbcb063 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2451,18 +2451,15 @@ 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;
> }
> + if (PageSwapCache(head))
> + set_page_private(page_tail, (unsigned long)head->private + tail);
> /* Page flags must be visible before we make the page non-compound. */
> smp_wmb();
> diff --git a/mm/memory.c b/mm/memory.c
> index d003076b218d..ff13242c1589 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3882,7 +3882,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> * changed.
> */
> if (unlikely(!folio_test_swapcache(folio) ||
> - page_private(page) != entry.val))
> + page_swap_entry(page).val != entry.val))
> goto out_page;
> /*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1f04debdc87a..ec7f8e6c9e48 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1647,7 +1647,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> */
> dec_mm_counter(mm, mm_counter(&folio->page));
> } else if (folio_test_anon(folio)) {
> - swp_entry_t entry = { .val = page_private(subpage) };
> + swp_entry_t entry = page_swap_entry(subpage);
> pte_t swp_pte;
> /*
> * Store the swap location in the pte.
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 01f15139b7d9..450819934e34 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -100,6 +100,7 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> folio_ref_add(folio, nr);
> folio_set_swapcache(folio);
> + folio_set_swap_entry(folio, entry);
> do {
> xas_lock_irq(&xas);
> @@ -113,7 +114,6 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> if (shadowp)
> *shadowp = old;
> }
> - set_page_private(folio_page(folio, i), entry.val + i);
> xas_store(&xas, folio);
> xas_next(&xas);
> }
> @@ -154,9 +154,9 @@ void __delete_from_swap_cache(struct folio *folio,
> for (i = 0; i < nr; i++) {
> void *entry = xas_store(&xas, shadow);
> VM_BUG_ON_PAGE(entry != folio, entry);
> - set_page_private(folio_page(folio, i), 0);
> xas_next(&xas);
> }
> + folio->private = 0;
I'd rather remove all direct reference to "private" for swap alongside, if
this would be the last spot (perhaps folio_set_swap_entry()).
> folio_clear_swapcache(folio);
> address_space->nrpages -= nr;
> __node_stat_mod_folio(folio, NR_FILE_PAGES, -nr);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index d46933adf789..bd9d904671b9 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3369,7 +3369,7 @@ struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> struct swap_info_struct *page_swap_info(struct page *page)
> {
> - swp_entry_t entry = { .val = page_private(page) };
> + swp_entry_t entry = page_swap_entry(page);
> return swp_swap_info(entry);
> }
> @@ -3384,7 +3384,7 @@ EXPORT_SYMBOL_GPL(swapcache_mapping);
> pgoff_t __page_file_index(struct page *page)
> {
> - swp_entry_t swap = { .val = page_private(page) };
> + swp_entry_t swap = page_swap_entry(page);
> return swp_offset(swap);
> }
> EXPORT_SYMBOL_GPL(__page_file_index);
> --
> 2.41.0
>
>
> --
> Cheers,
>
> David / dhildenb
>
--
Peter Xu
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