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dkim=pass header.d=fb.com header.s=facebook header.b=XHloiUtl; spf=none (imf05.hostedemail.com: domain of "prvs=5097f1d087=songliubraving@fb.com" has no SPF policy when checking 67.231.153.30) smtp.mailfrom="prvs=5097f1d087=songliubraving@fb.com"; dmarc=pass (policy=reject) header.from=fb.com X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: BEDD2100002 X-Stat-Signature: 1fmff18kufpjrjs4psw49kx7semh9yss X-HE-Tag: 1649453041-304800 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Apr 8, 2022, at 3:08 AM, Claudio Imbrenda wro= te: >=20 > On Thu, 7 Apr 2022 19:57:25 +0000 > Song Liu wrote: >=20 >> Hi Nicholas and Claudio,=20 >>=20 >>> On Apr 5, 2022, at 4:54 PM, Song Liu wrote: >>>=20 >>>> On Apr 5, 2022, at 12:07 AM, Christoph Hellwig wro= te: >>>>=20 >>>> On Fri, Apr 01, 2022 at 10:22:00PM +0000, Song Liu wrote: =20 >>>>>>> Please fix the underlying issues instead of papering over them and >>>>>>> creating a huge maintainance burden for others. =20 >>>>>=20 >>>>> After reading the code a little more, I wonder what would be best str= ategy.=20 >>>>> IIUC, most of the kernel is not ready for huge page backed vmalloc me= mory. >>>>> For example, all the module_alloc cannot work with huge pages at the = moment. >>>>> And the error Paul Menzel reported in drm_fb_helper.c will probably h= it=20 >>>>> powerpc with 5.17 kernel as-is? (trace attached below)=20 >>>>>=20 >>>>> Right now, we have VM_NO_HUGE_VMAP to let a user to opt out of huge p= ages.=20 >>>>> However, given there are so many users of vmalloc, vzalloc, etc., we= =20 >>>>> probably do need a flag for the user to opt-in?=20 >>>>>=20 >>>>> Does this make sense? Any recommendations are really appreciated. =20 >>>>=20 >>>> I think there is multiple aspects here: >>>>=20 >>>> - if we think that the kernel is not ready for hugepage backed vmalloc >>>> in general we need to disable it in powerpc for now. =20 >>>=20 >>> Nicholas and Claudio,=20 >>>=20 >>> What do you think about the status of hugepage backed vmalloc on powerp= c?=20 >>> I found module_alloc and kvm_s390_pv_alloc_vm() opt-out of huge pages. >>> But I am not aware of users that benefit from huge pages (except vfs ha= sh, >>> which was mentioned in 8abddd968a30). Does an opt-in flag (instead of=20 >>> current opt-out flag, VM_NO_HUGE_VMAP) make sense to you? =20 >>=20 >> Could you please share your comments on this? Specifically, does it make= =20 >> sense to replace VM_NO_HUGE_VMAP with an opt-in flag? If we think curren= t >> opt-out flag is better approach, what would be the best practice to find= =20 >> all the cases to opt-out? >=20 > An opt in flag would surely make sense, and it would be more backwards > compatible with existing code. That way each user can decide whether to > fix the code to allow for hugepages, if possible at all. For example, > the case you mentioned for s390 (kvm_s390_pv_alloc_vm) would not be > fixable, because of a hardware limitation (the whole area _must_ be > mapped with 4k pages) >=20 > If the consensus were to keep the current opt-put, then I guess each > user would have to check each usage of vmalloc and similar, and see if > anything breaks. To be honest, I think an opt-out would only be > possible after having the opt-in for a (long) while, when most users > would have fixed their code. >=20 > In short, I fully support opt-in. Thanks Claudio! I will prepare patches to replace VM_NO_HUGE_VMAP with an opt-in flag,=20 and use the new flag in BPF. Please let me know any comments/suggestions ont this direction.=20 Song