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Shutemov" , Jerome Glisse , Alistair Popple , Andrea Arcangeli , Mike Rapoport Subject: Re: [PATCH v2 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags Message-ID: References: <20210902201721.52796-1-peterx@redhat.com> <20210902201836.53605-1-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=VRuODBUR; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf25.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: C5D65B00008E X-Stat-Signature: 5zhfewunb7edh465bnsyys1t1wgghgci X-HE-Tag: 1631061795-850457 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 03, 2021 at 09:25:51AM +0200, David Hildenbrand wrote: > On 02.09.21 22:18, 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, it's confusing too on not explaining why passing in the details > > s/on// > > > pointer would 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 errornously skipped without being noticed. > > s/errornously/erroneously/ Will fix, thanks. > > > > > Actually very recently we introduced unmap_mapping_page() in 22061a1ffabd, I > > think that should also look into swap entries. Add a comment there. IOW, this > > patch will be a functional change to unmap_mapping_page() but hopefully in the > > right way to do it. I'll remove this paragraph, as explained elsewhere. > > > > 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). > > > > Cc: Kirill A. Shutemov > > Cc: Hugh Dickins > > Signed-off-by: Peter Xu > > --- > > 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 81e402a5fbc9..a7bcdb2ec956 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -1716,12 +1716,18 @@ 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)) > > Interestingly, this will also skip fake some swap entries (e.g., migration > entries but not private/exclusive entries). Maybe extend that documentation > a bit. > > ... but, looking into zap_pmd_range(), we don't care about "details" when > calling zap_huge_pmd(), which will zap pmd migration entries IIUC ... so > it's really unclear to me what the flag (and current behavior) really is and > what should be documented. Should we maybe really only care about "real" > swap entries? Indeed, I tried to look into it today and see why we wanted to skip swap entries but I failed to figure it out easily - it goes back to the 1st git commit of Linux. Maybe there'll be some experienced developer who knows the history, but before that I decided to just keep the behavior. The final goal of mine is to make the code clean and also prepares for the uffd-wp that will allow free-style use of "details" pointer, rather than have an implicit hint on "skip swap entry" then it'll be enough for this patch. > > Most probably I'm just missing something important. > > > + > > /* > > * 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 */ > > }; > > /* > > @@ -1737,6 +1743,16 @@ zap_skip_check_mapping(struct zap_details *details, struct page *page) > > return 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..4cb269ca8249 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)) > > @@ -3351,6 +3350,7 @@ void unmap_mapping_page(struct page *page) > > first_index = page->index; > > last_index = page->index + thp_nr_pages(page) - 1; > > + /* Keep ZAP_FLAG_SKIP_SWAP cleared because we're truncating */ As to keep the behavior, I shouldn't fiddle around with this, so I'll also attach ZAP_FLAG_SKIP_SWAP to unmap_mapping_page() too in v3. > > details.zap_mapping = mapping; > > details.single_page = page; Thanks, -- Peter Xu