On 07/12/2017 11:16 PM, Michal Hocko wrote: > On Wed 12-07-17 09:55:48, Mike Kravetz wrote: >> On 07/12/2017 04:46 AM, Michal Hocko wrote: >>> On Tue 11-07-17 11:23:19, Mike Kravetz wrote: >>>> On 07/11/2017 05:36 AM, Michal Hocko wrote: >>> [...] >>>>> Anyway the patch should fail with -EINVAL on private mappings as Kirill >>>>> already pointed out >>>> >>>> Yes. I think this should be a separate patch. As mentioned earlier, >>>> mremap today creates a new/additional private mapping if called in this >>>> way with old_size == 0. To me, this is a bug. >>> >>> Not only that. It clears existing ptes in the old mapping so the content >>> is lost. That is quite unexpected behavior. Now it is hard to assume >>> whether somebody relies on the behavior (I can easily imagine somebody >>> doing backup&clear in atomic way) so failing with EINVAL might break >>> userspace so I am not longer sure. Anyway this really needs to be >>> documented. >> >> I am pretty sure it does not clear ptes in the old mapping, or modify it >> in any way. Are you thinking they are cleared as part of the call to >> move_page_tables? Since old_size == 0 (len as passed to move_page_tables), >> the for loop in move_page_tables is not run and it doesn't do much of >> anything in this case. > > Dang. I have completely missed that we give old_len as the len > parameter. Then it is clear that this old_len == 0 trick never really > worked for MAP_PRIVATE because it simply fails the main invariant that > the content at the new location matches the old one. Care to send a > patch to clarify that and sent EINVAL or should I do it? Sent a patch (in separate e-mail thread) to return EINVAL for private mappings. >> If adding hugetlbfs support to memfd_create works out, I would like to >> see mremap(old_size == 0) support dropped. Nobody here (kernel mm >> development) seems to like it. However, as you note there may be somebody >> depending on this behavior. What would be the process for removing >> such support? AFAIK, it is not documented anywhere. If we do document >> the behavior, then we will certainly be stuck with it for a long time. > > I would rather document it than remove it. From the past we know that > there are users and my experience tells me that once something is used > it lives its life for ever basically. And moreover it is not like this > costs us any maintenance burden to support the hack. Just make it more > obvious so that we do not have to rediscover it each time. I will put together a patch to add a description of (old_size == 0) behavior to the man page. -- Mike Kravetz -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
