Re: THP backed thread stacks

[Hillf Danton <hdanton@sina.com>]

On 11 Mar 2023 12:24:58 +0000 William Kucharski <william.kucharski@oracle.com>
>> On Mar 10, 2023, at 04:25, David Hildenbrand <david@redhat.com> wrote:
>> On 10.03.23 02:40, William Kucharski wrote:
>>>> On Mar 9, 2023, at 17:05, Zach O'Keefe <zokeefe@google.com> wrote:
>>>>=20
>>>>> I think the hugepage alignment in their environment was somewhat luck.
>>>>> One suggestion made was to change stack size to avoid alignment and
>>>>> hugepage usage.  That 'works' but seems kind of hackish.
>>>>=20
>>>> That was my first thought, if the alignment was purely due to luck,
>>>> and not somebody manually specifying it. Agreed it's kind of hackish
>>>> if anyone can get bit by this by sheer luck.
>>> I don't agree it's "hackish" at all, but I go more into that below.
>>>>=20
>>>>> Also, David H pointed out the somewhat recent commit to align sufficie=
>ntly
>>>>> large mappings to THP boundaries.  This is going to make all stacks hu=
>ge
>>>>> page aligned.
>>>>=20
>>>> I think that change was reverted by Linus in commit 0ba09b173387
>>>> ("Revert "mm: align larger anonymous mappings on THP boundaries""),
>>>> until it's perf regressions were better understood -- and I haven't
>>>> seen a revamp of it.
>>> It's too bad it was reverted, though I understand the concerns regarding=
> it.
>>> From my point of view, if an address is properly aligned and a caller is
>>> asking for 2M+ to be mapped, it's going to be advantageous from a purely
>>> system-focused point of view to do that mapping with a THP.=20
>>=20
>> Just noting that, if user space requests multiple smaller mappings, and t=
>he kernel decides to all place them in the same PMD, all VMAs might get mer=
>ged and you end up with a properly aligned VMA where khugepaged would happi=
>ly place a THP.
>>=20
>> That case is, of course, different to the "user space asks for 2M+" mappi=
>ng case, but from khugepaged perspective they might look alike -- and it mi=
>ght be unclear if a THP is valuable or not (IOW maybe that THP could be bet=
>ter used somewhere else).
>
>That's a really, really good point.
>
>My general philosophy on the subject (if the address is aligned and the cal=
>ler is asking for a THP-sized allocation, why not map it with a THP if you =
>can) kind of falls apart when it's the system noticing it can coalesce a bu=
>nch of smaller allocations into one THP via khugepaged.
>
>Arguably it's the difference between the caller knowing it's asking for som=
>ething THP-sized on its behalf and the system deciding to remap a bunch of =
>disparate mappings using a THP because _it_ can.
>
>If we were to say allow a caller's request for a THP-sized allocation/mappi=
>ng take priority over those from khugepaged, it would not only be a major v=
>ector for abuse, it would also lead to completely indeterminate behavior ("=
>When I start my browser after a reboot I get a bunch of THPs, but after the=
> system's been up for a few weeks, I don't, how come?")

Given transparent_hugepage_flags, how would it be abused?  And indetermined?

>
>I don't have a good answer here.
>
>    -- Bill=