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Message-ID: <2576475.WBpAVSM2eX@nvdebian>
Date: Wed, 15 Sep 2021 12:25:07 +1000
From: Alistair Popple <apopple@...dia.com>
To: <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, <linux-mm@...ck.org>,
Peter Xu <peterx@...hat.com>
CC: Miaohe Lin <linmiaohe@...wei.com>,
David Hildenbrand <david@...hat.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Yang Shi <shy828301@...il.com>,
Matthew Wilcox <willy@...radead.org>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Jerome Glisse <jglisse@...hat.com>, <peterx@...hat.com>,
Liam Howlett <liam.howlett@...cle.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags
On Thursday, 9 September 2021 2:36:28 AM AEST Peter Xu wrote:
> Firstly, the comment in zap_pte_range() is misleading because it checks against
> details rather than check_mappings, so it's against what the code did.
>
> Meanwhile, there's no explicit reason why passing in the details pointer should
> mean to skip all swap entries. New user of zap_details could very possibly
> miss this fact if they don't read deep until zap_pte_range() because there's no
> comment at zap_details talking about it at all, so swap entries could be
> erroneously skipped without being noticed.
>
> This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
> but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
> "details" parameter: the caller should explicitly set this to skip swap
> entries, otherwise swap entries will always be considered (which should still
> be the major case here).
>
> We may want to look into when exactly we need ZAP_FLAG_SKIP_SWAP and we should
> have it in a synchronous manner, e.g., currently even if ZAP_FLAG_SKIP_SWAP is
> set we'll still look into swap pmds no matter what. But that should be a
> separate effort of this patch.
I didn't really follow what you mean by "synchronous" here, although the
explanation about pmds makes sense so it's probably just terminology.
> The flag introduced in this patch will be a preparation for more bits defined
> in the future, e.g., for a new bit in flag to show whether to persist the
> upcoming uffd-wp bit in pgtable entries.
That's kind of the problem. The patch itself looks correct to me however as
mentioned it is mostly reverting a previous cleanup and it's hard to tell why
that's justified without the subsequent patches. Perhaps it makes the usage of
zap_details a bit clearer, but a comment also would with less code.
I know you want to try and shrink the uffd-wp series but I think this patch
might be easier to review if it was included as part of that series.
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> include/linux/mm.h | 16 ++++++++++++++++
> mm/memory.c | 6 +++---
> 2 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ed44f31615d9..beb784ce35b9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1717,12 +1717,18 @@ static inline bool can_do_mlock(void) { return false; }
> extern int user_shm_lock(size_t, struct ucounts *);
> extern void user_shm_unlock(size_t, struct ucounts *);
>
> +typedef unsigned int __bitwise zap_flags_t;
> +
> +/* Whether to skip zapping swap entries */
> +#define ZAP_FLAG_SKIP_SWAP ((__force zap_flags_t) BIT(0))
> +
> /*
> * Parameter block passed down to zap_pte_range in exceptional cases.
> */
> struct zap_details {
> struct address_space *zap_mapping; /* Check page->mapping if set */
> struct page *single_page; /* Locked page to be unmapped */
> + zap_flags_t zap_flags; /* Extra flags for zapping */
> };
>
> /*
> @@ -1739,6 +1745,16 @@ zap_skip_check_mapping(struct zap_details *details, struct page *page)
> (details->zap_mapping != page_rmapping(page));
> }
>
> +/* Return true if skip swap entries, false otherwise */
> +static inline bool
> +zap_skip_swap(struct zap_details *details)
> +{
> + if (!details)
> + return false;
> +
> + return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
> +}
> +
> struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> pte_t pte);
> struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> diff --git a/mm/memory.c b/mm/memory.c
> index e5ee8399d270..26e37bef1888 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1379,8 +1379,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> continue;
> }
>
> - /* If details->check_mapping, we leave swap entries. */
> - if (unlikely(details))
> + if (unlikely(zap_skip_swap(details)))
> continue;
>
> if (!non_swap_entry(entry))
> @@ -3353,6 +3352,7 @@ void unmap_mapping_page(struct page *page)
>
> details.zap_mapping = mapping;
> details.single_page = page;
> + details.zap_flags = ZAP_FLAG_SKIP_SWAP;
>
> i_mmap_lock_write(mapping);
> if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
> @@ -3377,7 +3377,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
> pgoff_t nr, bool even_cows)
> {
> pgoff_t first_index = start, last_index = start + nr - 1;
> - struct zap_details details = { };
> + struct zap_details details = { .zap_flags = ZAP_FLAG_SKIP_SWAP };
>
> details.zap_mapping = even_cows ? NULL : mapping;
> if (last_index < first_index)
>
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