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01 May 2020 09:32:46 -0700 IronPort-SDR: IRlEnw96PyW2wS0xq/MI/Ft7/+0Pp51LZQsrLWOgqrKr4uogpI7iSja65OD1MiLWM19MAn6TgZ Kj4V8cTQry3Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,340,1583222400"; d="scan'208";a="262131924" Received: from vjaiswa1-mobl.amr.corp.intel.com (HELO [10.252.137.219]) ([10.252.137.219]) by orsmga006.jf.intel.com with ESMTP; 01 May 2020 09:32:45 -0700 Subject: Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages To: Christian Borntraeger , Claudio Imbrenda , viro@zeniv.linux.org.uk Cc: david@redhat.com, akpm@linux-foundation.org, aarcange@redhat.com, linux-mm@kvack.org, frankja@linux.ibm.com, sfr@canb.auug.org.au, jhubbard@nvidia.com, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, jack@suse.cz, kirill@shutemov.name, peterz@infradead.org, sean.j.christopherson@intel.com, Ulrich.Weigand@de.ibm.com References: <20200430143825.3534128-1-imbrenda@linux.ibm.com> <1a3f5107-9847-73d4-5059-c6ef9d293551@de.ibm.com> <3d379d9e-241c-ef3b-dcef-20fdd3b8740d@de.ibm.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <3d379d9e-241c-ef3b-dcef-20fdd3b8740d@de.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable 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: On 5/1/20 12:18 AM, Christian Borntraeger wrote: >> unlock_page(); >> get_page(); >> // ^ OK because I have a ref >> // do DMA on inaccessible page >> >> Because the make_secure_pte() code isn't looking for a *specific* >> 'expected' value, it has no way of noticing that the extra ref snuck i= n >> there. > I think the expected calcution is actually doing that,giving back the m= inimum > value when no one else has any references that are valid for I/O. >=20 > But I might not have understood what you are trying to tell me? I was wrong. I was looking at migrate_page_move_mapping(): > int expected_count =3D expected_page_refs(mapping, page) + extr= a_count; ... > xas_lock_irq(&xas); > if (page_count(page) !=3D expected_count || xas_load(&xas) !=3D= page) { > xas_unlock_irq(&xas); > return -EAGAIN; > } >=20 > if (!page_ref_freeze(page, expected_count)) { > xas_unlock_irq(&xas); > return -EAGAIN; > } I saw the check for page_count(page) *and* the page_ref_freeze() call. My assumption was that both were needed. My assumption was wrong. (I think the migrate_page_move_mapping() code may actually be doing a superfluous check.) The larger point, though, is that the s390 code ensures no extra references exist upon entering make_secure_pte(), but it still has no mechanism to prevent future, new references to page cache pages from being created. The one existing user of expected_page_refs() freezes the refs then *removes* the page from the page cache (that's what the xas_lock_irq() is for). That stops *new* refs from being acquired. The s390 code is missing an equivalent mechanism. One example: page_freeze_refs(); // page->_count=3D=3D0 now find_get_page(); // ^ sees a "freed" page page_unfreeze_refs(); find_get_page() will either fail to *find* the page because it will see page->_refcount=3D=3D0 think it is freed (not great), or it will VM_BUG_ON_PAGE() in __page_cache_add_speculative(). My bigger point is that this patches doesn't systematically stop finding page cache pages that are arch-inaccessible. This patch hits *one* of those sites.