Messages in this thread | | | Subject | Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility | From | Yang Shi <> | Date | Thu, 6 Jun 2019 11:59:21 -0700 |
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On 5/7/19 10:10 AM, Yang Shi wrote: > > > 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.
[Forgot cc to mailing lists]
Hi guys,
How should we move forward for this one? Make the sysfs knob (/sys/kernel/mm/transparent_hugepage/enabled) to be global for both anonymous and tmpfs? Or just treat shmem objects separately from anon memory then fix the false-negative of THP eligibility by this patch?
> >> >> Kirill, Hugh othe folks? >
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