Messages in this thread | | | Subject | Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility | From | Yang Shi <> | Date | Tue, 7 May 2019 10:10:33 -0700 |
| |
On 5/7/19 3:47 AM, Michal Hocko wrote: > [Hmm, I thought, Hugh was CCed] > > On Mon 06-05-19 16:37:42, Yang Shi wrote: >> >> On 4/28/19 12:13 PM, Yang Shi wrote: >>> >>> On 4/23/19 10:52 AM, Michal Hocko wrote: >>>> On Wed 24-04-19 00:43:01, Yang Shi wrote: >>>>> The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility >>>>> for each >>>>> vma") introduced THPeligible bit for processes' smaps. But, when >>>>> checking >>>>> the eligibility for shmem vma, __transparent_hugepage_enabled() is >>>>> called to override the result from shmem_huge_enabled(). It may result >>>>> in the anonymous vma's THP flag override shmem's. For example, >>>>> running a >>>>> simple test which create THP for shmem, but with anonymous THP >>>>> disabled, >>>>> when reading the process's smaps, it may show: >>>>> >>>>> 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test >>>>> Size: 4096 kB >>>>> ... >>>>> [snip] >>>>> ... >>>>> ShmemPmdMapped: 4096 kB >>>>> ... >>>>> [snip] >>>>> ... >>>>> THPeligible: 0 >>>>> >>>>> And, /proc/meminfo does show THP allocated and PMD mapped too: >>>>> >>>>> ShmemHugePages: 4096 kB >>>>> ShmemPmdMapped: 4096 kB >>>>> >>>>> This doesn't make too much sense. The anonymous THP flag should not >>>>> intervene shmem THP. Calling shmem_huge_enabled() with checking >>>>> MMF_DISABLE_THP sounds good enough. And, we could skip stack and >>>>> dax vma check since we already checked if the vma is shmem already. >>>> Kirill, can we get a confirmation that this is really intended behavior >>>> rather than an omission please? Is this documented? What is a global >>>> knob to simply disable THP system wise? >>> Hi Kirill, >>> >>> Ping. Any comment? >> Talked with Kirill at LSFMM, it sounds this is kind of intended behavior >> according to him. But, we all agree it looks inconsistent. >> >> So, we may have two options: >> - Just fix the false negative issue as what the patch does >> - Change the behavior to make it more consistent >> >> I'm not sure whether anyone relies on the behavior explicitly or implicitly >> or not. > Well, I would be certainly more happy with a more consistent behavior. > Talked to Hugh at LSFMM about this and he finds treating shmem objects > separately from the anonymous memory. And that is already the case > partially when each mount point might have its own setup. So the primary > question is whether we need a one global knob to controll all THP > allocations. One argument to have that is that it might be helpful to > for an admin to simply disable source of THP at a single place rather > than crawling over all shmem mount points and remount them. Especially > in environments where shmem points are mounted in a container by a > non-root. Why would somebody wanted something like that? One example > would be to temporarily workaround high order allocations issues which > we have seen non trivial amount of in the past and we are likely not at > the end of the tunel.
Shmem has a global control for such use. Setting shmem_enabled to "force" or "deny" would enable or disable THP for shmem globally, including non-fs objects, i.e. memfd, SYS V shmem, etc.
> > That being said I would be in favor of treating the global sysfs knob to > be global for all THP allocations. I will not push back on that if there > is a general consensus that shmem and fs in general are a different > class of objects and a single global control is not desirable for > whatever reasons.
OK, we need more inputs from Kirill, Hugh and other folks.
> > Kirill, Hugh othe folks?
| |