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Sat, 11 Dec 2021 07:16:21 -0800 (PST) Subject: Re: [PATCH V2] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask From: Li Xinhai To: linux-mm@kvack.org Cc: akpm@linux-foundation.org References: <20211211151014.650778-1-lixinhai.lxh@gmail.com> Message-ID: <5c0f5b5c-7c22-a7ab-4add-fa6bd11f7af8@gmail.com> Date: Sat, 11 Dec 2021 23:16:13 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <20211211151014.650778-1-lixinhai.lxh@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E83E9100002 X-Stat-Signature: ow857qe9xm475cnzz7ioq1rwcqfp9zqs Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=MHmbifto; spf=pass (imf05.hostedemail.com: domain of lixinhai.lxh@gmail.com designates 209.85.167.42 as permitted sender) smtp.mailfrom=lixinhai.lxh@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-HE-Tag: 1639235782-616299 X-Bogosity: Ham, tests=bogofilter, spamicity=0.025260, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 12/11/21 11:10 PM, Li Xinhai wrote: > When BUG_ON check for THP migration entry, the exsiting code only check > thp_migration_supported case, but not for !thp_migration_supported case. > To make the BUG_ON check consistent, we need catch both cases. > > Move the BUG_ON check one step eariler, because if the bug happen we > should know it instead of depend on FOLL_MIGRATION been used by caller. > > Because pmdval instead of *pmd is read by the is_pmd_migration_entry() > check, the existing code don't help to avoid useless locking within > pmd_migration_entry_wait(), so remove that check. > > Signed-off-by: Li Xinhai > --- V1->V2: Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments for it. > mm/gup.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 2c51e9748a6a..94d0e586ca0b 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > } > retry: > if (!pmd_present(pmdval)) { > + /* > + * Should never reach here, if thp migration is not supported; > + * Otherwise, it must be a thp miration entry. > + */ > + VM_BUG_ON(!thp_migration_supported() || > + !is_pmd_migration_entry(pmdval)); > + > if (likely(!(flags & FOLL_MIGRATION))) > return no_page_table(vma, flags); > - VM_BUG_ON(thp_migration_supported() && > - !is_pmd_migration_entry(pmdval)); > - if (is_pmd_migration_entry(pmdval)) > - pmd_migration_entry_wait(mm, pmd); > + > + pmd_migration_entry_wait(mm, pmd); > pmdval = READ_ONCE(*pmd); > /* > * MADV_DONTNEED may convert the pmd to null because >