From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta7.messagelabs.com (mail6.bemta7.messagelabs.com [216.82.255.55]) by kanga.kvack.org (Postfix) with ESMTP id 7EB386B0069 for ; Sun, 20 Nov 2011 23:16:21 -0500 (EST) Received: by iaek3 with SMTP id k3so8990220iae.14 for ; Sun, 20 Nov 2011 20:16:17 -0800 (PST) Date: Sun, 20 Nov 2011 20:16:06 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH] ksm: use FAULT_FLAG_ALLOW_RETRY in breaking COW In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Hillf Danton Cc: Michel Lespinasse , Andrea Arcangeli , Andrew Morton , linux-mm@kvack.org, LKML , Michal Hocko On Sat, 19 Nov 2011, Hillf Danton wrote: > The flag, FAULT_FLAG_ALLOW_RETRY, was introduced by the patch, > > mm: retry page fault when blocking on disk transfer > commit: d065bd810b6deb67d4897a14bfe21f8eb526ba99 > > for reducing mmap_sem hold times that are caused by waiting for disk > transfers when accessing file mapped VMAs. > > To break COW, handle_mm_fault() is repeated with mmap_sem held, where > the introduced flag could be used again. > > The straight way is to add changes in break_ksm(), but the function could be > under write-mode mmap_sem, so it has to be dupilcated. > > Signed-off-by: Hillf Danton Thank you for making the patch; but unless I'm mistaken - please correct me if so - I think it's better to keep break_cow() simple than add special FAULT_FLAG_ALLOW_RETRY handling there. Do you have any evidence that its down_read of mmap_sem is a problem in some workload? I sense that you're using it "because it's there". I'm sceptical on several grounds. One, break_cow() is itself only called on an "error" path: not really an error, but when KSM's bet that it can merge pages turns out to be wrong before it can complete the merge; not a rare case, but not on the hot path. Two, break_ksm()'s loop is required for correctness, but it is a rare case that it actually needs to go round a second time. The typical case it's needed (am I forgetting a more common one?) is when userspace access flips a pte bit in between handle_pte_fault() noting faulting pte, and the chosen fault handler checking pte_same() before committing to its action. With the page marked PageKsm, yet not in the stable tree, even page reclaim is unable to interfere. Three, FAULT_FLAG_ALLOW_RETRY is acted upon only in lock_page_or_retry(), which is called only from filemap_fault() (not the case here since we don't consider file pages for conversion to PageKsm) or do_swap_page(); yet the fault we're provoking would be handled by do_wp_page(). Four, lock_page_or_retry() is called in those places when there's a possibility that the page is being read in from disk, to drop the mmap_sem across the slow I/O. There is no precedent for dropping mmap_sem here while allocating a new page, nor when pte_same() fails: in the former case it could only be a win when the system is already slowed by memory pressure, in the latter case there's little point, since mmap_sem would be reacquired in a moment. I think that amounts to a genial Nack! Hugh > --- > > --- a/mm/ksm.c Sat Nov 19 16:08:10 2011 > +++ b/mm/ksm.c Sat Nov 19 19:33:49 2011 > @@ -394,7 +394,31 @@ static void break_cow(struct rmap_item * > goto out; > if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma) > goto out; > - break_ksm(vma, addr); > + for (;;) { > + struct page *page; > + int ret; > + > + page = follow_page(vma, addr, FOLL_GET); > + if (IS_ERR_OR_NULL(page)) > + break; > + > + if (PageKsm(page)) > + ret = handle_mm_fault(mm, vma, addr, > + FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_WRITE); > + else > + ret = VM_FAULT_WRITE; > + > + put_page(page); > + > + if (!(ret & (VM_FAULT_WRITE|VM_FAULT_SIGBUS|VM_FAULT_OOM))) { > + if (ret & VM_FAULT_RETRY) > + down_read(&mm->mmap_sem); > + } else { > + if (ret & VM_FAULT_RETRY) > + return; > + break; > + } > + } > out: > up_read(&mm->mmap_sem); > } -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org