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Date: Mon, 21 Jun 2021 12:16:50 -0400
From: Peter Xu <peterx@...hat.com>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Andrea Arcangeli <aarcange@...hat.com>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Nadav Amit <nadav.amit@...il.com>,
Hugh Dickins <hughd@...gle.com>,
Jerome Glisse <jglisse@...hat.com>,
Jason Gunthorpe <jgg@...pe.ca>,
Andrew Morton <akpm@...ux-foundation.org>,
Miaohe Lin <linmiaohe@...wei.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Matthew Wilcox <willy@...radead.org>,
Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3 08/27] mm: Introduce zap_details.zap_flags
On Mon, Jun 21, 2021 at 10:09:00PM +1000, Alistair Popple wrote:
> On Friday, 28 May 2021 6:21:30 AM AEST Peter Xu wrote:
> > Instead of trying to introduce one variable for every new zap_details fields,
> > let's introduce a flag so that it can start to encode true/false informations.
> >
> > Let's start to use this flag first to clean up the only check_mapping variable.
> > Firstly, the name "check_mapping" implies this is a "boolean", but actually it
> > stores the mapping inside, just in a way that it won't be set if we don't want
> > to check the mapping.
> >
> > To make things clearer, introduce the 1st zap flag ZAP_FLAG_CHECK_MAPPING, so
> > that we only check against the mapping if this bit set. At the same time, we
> > can rename check_mapping into zap_mapping and set it always.
> >
> > Since at it, introduce another helper zap_check_mapping_skip() and use it in
> > zap_pte_range() properly.
> >
> > Some old comments have been removed in zap_pte_range() because they're
> > duplicated, and since now we're with ZAP_FLAG_CHECK_MAPPING flag, it'll be very
> > easy to grep this information by simply grepping the flag.
> >
> > It'll also make life easier when we want to e.g. pass in zap_flags into the
> > callers like unmap_mapping_pages() (instead of adding new booleans besides the
> > even_cows parameter).
> >
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> > include/linux/mm.h | 19 ++++++++++++++++++-
> > mm/memory.c | 31 ++++++++-----------------------
> > 2 files changed, 26 insertions(+), 24 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index db155be8e66c..52d3ef2ed753 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1721,13 +1721,30 @@ static inline bool can_do_mlock(void) { return false; }
> > extern int user_shm_lock(size_t, struct user_struct *);
> > extern void user_shm_unlock(size_t, struct user_struct *);
> >
> > +/* Whether to check page->mapping when zapping */
> > +#define ZAP_FLAG_CHECK_MAPPING BIT(0)
> > +
> > /*
> > * Parameter block passed down to zap_pte_range in exceptional cases.
> > */
> > struct zap_details {
> > - struct address_space *check_mapping; /* Check page->mapping if set */
> > + struct address_space *zap_mapping;
> > + unsigned long zap_flags;
> > };
> >
> > +/* Return true if skip zapping this page, false otherwise */
> > +static inline bool
> > +zap_check_mapping_skip(struct zap_details *details, struct page *page)
> > +{
> > + if (!details || !page)
> > + return false;
> > +
> > + if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
> > + return false;
[1]
> > +
> > + return details->zap_mapping != page_rmapping(page);
>
> I doubt this matters in practice, but there is a slight behaviour change
> here that might be worth checking. Previously this check was equivalent
> to:
>
> details->zap_mapping && details->zap_mapping != page_rmapping(page)
Yes; IMHO "details->zap_mapping" is just replaced by the check at [1].
For example, there's only one real user of this mapping check, which is
unmap_mapping_pages() below [2].
With the old code, we have:
details.check_mapping = even_cows ? NULL : mapping;
So "details->zap_mapping" is only true if "!even_cows".
With the new code, we'll have:
if (!even_cows)
details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
So ZAP_FLAG_CHECK_MAPPING is only set if "!even_cows", while that's what we
check exactly at [1].
>
> Otherwise I think this looks good.
>
> > +}
> > +
> > 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 27cf8a6375c6..c9dc4e9e05b5 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1330,16 +1330,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> > struct page *page;
> >
> > page = vm_normal_page(vma, addr, ptent);
> > - if (unlikely(details) && page) {
> > - /*
> > - * unmap_shared_mapping_pages() wants to
> > - * invalidate cache without truncating:
> > - * unmap shared but keep private pages.
> > - */
> > - if (details->check_mapping &&
> > - details->check_mapping != page_rmapping(page))
> > - continue;
> > - }
> > + if (unlikely(zap_check_mapping_skip(details, page)))
> > + continue;
> > ptent = ptep_get_and_clear_full(mm, addr, pte,
> > tlb->fullmm);
> > tlb_remove_tlb_entry(tlb, pte, addr);
> > @@ -1372,17 +1364,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> > is_device_exclusive_entry(entry)) {
> > struct page *page = pfn_swap_entry_to_page(entry);
> >
> > - if (unlikely(details && details->check_mapping)) {
> > - /*
> > - * unmap_shared_mapping_pages() wants to
> > - * invalidate cache without truncating:
> > - * unmap shared but keep private pages.
> > - */
> > - if (details->check_mapping !=
> > - page_rmapping(page))
> > - continue;
> > - }
> > -
> > + if (unlikely(zap_check_mapping_skip(details, page)))
> > + continue;
> > pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > rss[mm_counter(page)]--;
> >
> > @@ -3345,9 +3328,11 @@ 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_mapping = mapping };
> > +
> > + if (!even_cows)
> > + details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
> >
> > - details.check_mapping = even_cows ? NULL : mapping;
[2]
> > if (last_index < first_index)
> > last_index = ULONG_MAX;
Thanks,
--
Peter Xu
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