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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id gd27-20020a05622a5c1b00b0039c37a7914csm2840053qtb.23.2022.10.26.14.59.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Oct 2022 14:59:45 -0700 (PDT) Date: Wed, 26 Oct 2022 17:59:43 -0400 From: Peter Xu To: Mike Kravetz Subject: Re: [PATCH v3] hugetlb: simplify hugetlb handling in follow_page_mask Message-ID: References: <20220919021348.22151-1-mike.kravetz@oracle.com> MIME-Version: 1.0 In-Reply-To: <20220919021348.22151-1-mike.kravetz@oracle.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Naoya Horiguchi , linux-ia64@vger.kernel.org, David Hildenbrand , "Aneesh Kumar K . V" , Muchun Song , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Baolin Wang , Andrew Morton , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Hi, Mike, On Sun, Sep 18, 2022 at 07:13:48PM -0700, Mike Kravetz wrote: > +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma, > + unsigned long address, unsigned int flags) > +{ > + struct hstate *h = hstate_vma(vma); > + struct mm_struct *mm = vma->vm_mm; > + unsigned long haddr = address & huge_page_mask(h); > + struct page *page = NULL; > + spinlock_t *ptl; > + pte_t *pte, entry; > + > + /* > + * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via > + * follow_hugetlb_page(). > + */ > + if (WARN_ON_ONCE(flags & FOLL_PIN)) > + return NULL; > + > +retry: > + /* > + * vma lock prevents racing with another thread doing a pmd unshare. > + * This keeps pte as returned by huge_pte_offset valid. > + */ > + hugetlb_vma_lock_read(vma); I'm not sure whether it's okay to take a rwsem here, as the code can be called by e.g. FOLL_NOWAIT? I'm wondering whether it's fine to just drop this anyway, just always walk it lockless. IIUC gup callers should be safe here because the worst case is the caller will fetch a wrong page, but then it should be invalidated very soon with mmu notifiers. One thing worth mention is that pmd unshare should never free a pgtable page. IIUC it's also the same as fast-gup - afaiu we don't take the read vma lock in fast-gup too but I also think it's safe. But I hope I didn't miss something. -- Peter Xu