From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2D730C27C7A for ; Thu, 17 Aug 2023 12:19:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350753AbjHQMTL (ORCPT ); Thu, 17 Aug 2023 08:19:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42374 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350825AbjHQMSu (ORCPT ); Thu, 17 Aug 2023 08:18:50 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 942D02D50 for ; Thu, 17 Aug 2023 05:18:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=+MuyupV+mEo0a8sy4kXCfy1lN0SMoP5voFKVS9XKlr0=; b=eyMaKs5jzocpH7Imu9V+zeeKOE eYFowVnB5AbsXcFF2s8nHH46fLC7W1yPJXDie60Rsbes8bAoC5ODQgpuzG7a5guV3FBAUHjLUShag Vq8fWt5/2RLVoR345mT63PqPgNllkQFJu7t6QI2O8ud9i1KZ0IvawmdUTtCouSCr+nWJc1TT1TjWF II3H1UOkxaMe0UljaN08VPj6LNE2KDsCz+yueer4M7hBnQRWf2p+otlI9arzAMA9gsjYXbyvYwdJg +86YmG94ry+cS2HeWBbB4/Y5W2f+OCSYCD1CRZoj9P6h2ERFobUgr9bnfcn/OnOPZ7tYULDB3n/GV OCuiURGA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qWbxM-003Bnx-AV; Thu, 17 Aug 2023 12:18:28 +0000 Date: Thu, 17 Aug 2023 13:18:28 +0100 From: Matthew Wilcox To: Zach O'Keefe Cc: Saurabh Singh Sengar , Dan Williams , "linux-mm@kvack.org" , Yang Shi , "linux-kernel@vger.kernel.org" Subject: Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()" Message-ID: References: <20230812210053.2325091-1-zokeefe@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 16, 2023 at 02:31:06PM -0700, Zach O'Keefe wrote: > On Mon, Aug 14, 2023 at 7:24 PM Matthew Wilcox wrote: > > So if we find a large folio that is PMD mappable, and there's nothing > > at vmf->pmd, we install a PMD-sized mapping at that spot. If that > > fails, we install the preallocated PTE table at vmf->pmd and continue to > > trying set one or more PTEs to satisfy this page fault. > > Aha! I see. I did not expect ->fault() to have this logic, as I had > incorrectly thought (aka assumed) the pmd vs pte-mapping logic split > at create_huge_pmd(); i.e. do_huge_pmd_anonymous_page(), or > ->huge_fault(), or fallback to pte-mapping. It seems very weird to me > that hugepage_vma_check() "artificially" says "no" to file and shmem > along the fault path, so they can go and do their own thing in > ->fault(). Wow, hugepage_vma_check() is a very complicated function. I'm glad I ignored it! > IIUC then, there is a bug in smaps THPeligible code when > CONFIG_READ_ONLY_THP_FOR_FS is not set. Not obvious, but apparently > this config is (according to it's Kconfig desc) khugepaged-only, so it > should be fine for it to be disabled, yet allow > do_sync_mmap_readahead() to install a pmd for file-backed memory. > hugepage_vma_check() will need to be patched to fix this. I guess so ... > But I have a larger question for you: should we care about > /sys/kernel/mm/transparent_hugepage/enabled for file-fault? We > currently don't. Seems weird that we can transparently get a hugepage > when THP="never". Also, if THP="always", we might as well skip the > VM_HUGEPAGE check, and try the final pmd install (and save khugepaged > the trouble of attempting it later). I deliberately ignored the humungous complexity of the THP options. They're overgrown and make my brain hurt. Instead, large folios are adaptive; they observe the behaviour of the user program and choose based on history what to do. This is far superior to having a sysadmin tell us what to do!