From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-gg0-f182.google.com (mail-gg0-f182.google.com [209.85.161.182]) by kanga.kvack.org (Postfix) with ESMTP id EB1826B00AF for ; Tue, 21 Jan 2014 18:24:13 -0500 (EST) Received: by mail-gg0-f182.google.com with SMTP id e27so2833909gga.13 for ; Tue, 21 Jan 2014 15:24:13 -0800 (PST) Received: from mail-gg0-x233.google.com (mail-gg0-x233.google.com [2607:f8b0:4002:c02::233]) by mx.google.com with ESMTPS id 21si5678452yhx.181.2014.01.21.15.24.11 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 21 Jan 2014 15:24:12 -0800 (PST) Received: by mail-gg0-f179.google.com with SMTP id e5so2817700ggh.38 for ; Tue, 21 Jan 2014 15:24:11 -0800 (PST) Date: Tue, 21 Jan 2014 15:24:08 -0800 (PST) From: David Rientjes Subject: Re: [BUG] mm: thp: hugepage_vma_check has a blind spot In-Reply-To: <1390345671-136133-1-git-send-email-athorlton@sgi.com> Message-ID: References: <1390345671-136133-1-git-send-email-athorlton@sgi.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Alex Thorlton Cc: linux-kernel@vger.kernel.org, Andrew Morton , Mel Gorman , Rik van Riel , Ingo Molnar , Peter Zijlstra , linux-mm@kvack.org On Tue, 21 Jan 2014, Alex Thorlton wrote: > hugepage_vma_check is called during khugepaged_scan_mm_slot to ensure > that khugepaged doesn't try to allocate THPs in vmas where they are > disallowed, either due to THPs being disabled system-wide, or through > MADV_NOHUGEPAGE. > > The logic that hugepage_vma_check uses doesn't seem to cover all cases, > in my opinion. Looking at the original code: > > if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) || > (vma->vm_flags & VM_NOHUGEPAGE)) > > We can see that it's possible to have THP disabled system-wide, but still > receive THPs in this vma. It seems that it's assumed that just because > khugepaged_always == false, TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG must be > set, which is not the case. We could have VM_HUGEPAGE set, but have THP > set to "never" system-wide, in which case, the condition presented in the > if will evaluate to false, and (provided the other checks pass) we can > end up giving out a THP even though the behavior is set to "never." > You should be able to add a BUG_ON(current != khugepaged_thread); here since khugepaged is supposed to be the only caller to the function. > While we do properly check these flags in khugepaged_has_work, it looks > like it's possible to sleep after we check khugepaged_hask_work, but > before hugepage_vma_check, during which time, hugepages could have been > disabled system-wide, in which case, we could hand out THPs when we > shouldn't be. > You're talking about when thp is set to "never" and before khugepaged has stopped, correct? That doesn't seem like a bug to me or anything that needs to be fixed, the sysfs knob could be switched even after hugepage_vma_check() is called and before a hugepage is actually collapsed so you have the same race. The only thing that's guaranteed is that, upon writing "never" to /sys/kernel/mm/transparent_hugepage/enabled, no more thp memory will be collapsed after khugepaged has stopped. -- 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/ . Don't email: email@kvack.org