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1;DB6PR0801MB1733;20:J5IAJCdMqd/qfYUNIiMjAn6L5bWy6urdfWQc52YW6NEyiYZqbO0rzKGFkLJPvrwfbO99DObgHqsD17EXLmvyjQeaegb/bpQEpNgsuzNNkDM5pPkwDsQ8AK89R6UVFP74GTwj9HO2bqP3Rmo9NGmf3/WM+53nnrS0MDBfMYSJV3c= X-OriginatorOrg: virtuozzo.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Jun 2017 10:58:46.2470 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB6PR0801MB1733 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2677 Lines: 69 On 06/22/2017 04:07 AM, Hugh Dickins wrote: > On Wed, 21 Jun 2017, Linus Torvalds wrote: >> On Wed, Jun 21, 2017 at 1:56 PM, Oleg Nesterov wrote: >>> >>> I understand. My point is that this check was invalidated by stack-guard-page >>> a long ago, and this means that we add the user-visible change now. >> >> Yeah. I guess we could consider it an *old* regression that got fixed, >> but if people started relying on the regression... >> >>>> Do you have a pointer to the report for this regression? I must have missed it. >>> >>> See http://marc.info/?t=149794523000001&r=1&w=2 >> >> Ok. >> >> And thinking about it, while that is a silly test-case, the notion of >> "create top-down segment, then start populating it _before_ moving the >> stack pointer into it" is actually perfectly valid. >> >> So I guess checking against the stack pointer is wrong in that case - >> at least if the stack pointer isn't inside that vma to begin with. >> >> So yes, removing that check looks like the right thing to do for now. >> >> Do you want to send me the patch if you already have a commit message etc? > > I have a bit of a bad feeling about this. > > Perhaps it's just sentimental attachment to all those weird > and ancient stack pointer checks in arch//fault.c. > > We have been inconsistent: cris frv m32r m68k microblaze mn10300 > openrisc powerpc tile um x86 have such checks, the others don't. > So that's a good reason to delete them. > > But at least at the moment those checks impose some sanity: > just a page less than we had imagined for several years. > Once we remove them, they cannot go back. Should we now > complicate them with an extra page of slop? > > I'm not entirely persuaded by your pre-population argument: > it's perfectly possible to prepare a MAP_GROWSDOWN area with > an initial size, that's populated in a normal way, before handing > off for stack expansion - isn't it? > > I'd be interested to hear more about that (redhat internal) bug > report that Oleg mentions: whether it gives stronger grounds for > making this sudden change than the CRIU testcase. Well, if all the deal is in CRIU testcase - it can be easily reworked. The question - will it break anything else? Maybe it's better to disable this check on the release and enable it back for v4.13 kernel, so if it'll break some user-space, it'll be caught on linux-next. > > I can go ahead and create a patch if Oleg is not there at the > moment - but I might prefer his or your name on it - particularly > if we're rushing it in before consulting the arch maintainers > whose work we would be deleting. > > Queasily, > Hugh > -- Dmitry