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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 Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-ia64@vger.kernel.org, Baolin Wang , David Hildenbrand , Christophe Leroy , "Aneesh Kumar K . V" , Naoya Horiguchi , Michael Ellerman , Muchun Song , Andrew Morton 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 ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666821590; a=rsa-sha256; cv=none; b=dVYPjbbVe5DqsgD42vCjBXtDQAW6Sy27iN4vg/mlxbqJetMICxjaoYkyFyTRcX0IRx625T rXEmNcIxtR+APhc/R4VJR2ipNzlacomjwd52TQfwwKmA2lKslNN0CCV21xghOSEldvkpiT yPdOFQHRAxodmhjfOEq++lO3OxmfOZU= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=KZXaVzkL; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf16.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666821590; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=QNXqWwBEWZqwUkNcA15/4+3p4mEEOSR1ir8+lCD/vgI=; b=LNwMw2TG4U4oU3Zeh7Revr0C4Kq98RtVoPrZ5nbEOYnLLOEqspM5S64ckd4V1ub5b0muZJ ebHDsQglFGFiOtqTo+dXXUl0V5BIVBlErGpDyond91TlNxef+lr524Y2fOGOsXrCR2ILDa A4vs+4HL1hTICTY8LSAuGAOm2/6pdUc= X-Stat-Signature: qtp95wyycspg55qubx3ixzsh5ma3fd1a X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: BA237180005 Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=KZXaVzkL; dmarc=pass (policy=none) header.from=redhat.com; spf=pass (imf16.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com X-HE-Tag: 1666821590-452842 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: 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