On 25/07/2025 14:08, David Hildenbrand wrote: > On 25.07.25 00:27, Usama Arif wrote: >> >>> Hi! >>> >>>> >>>> Over here, with MMF_DISABLE_THP_EXCEPT_ADVISED, MADV_HUGEPAGE will succeed as vm_flags has >>>> VM_HUGEPAGE set, but MADV_COLLAPSE will fail to give a hugepage (as VM_HUGEPAGE is not set >>>> and MMF_DISABLE_THP_EXCEPT_ADVISED is set) which I feel might not be the right behaviour >>>> as MADV_COLLAPSE is "advise" and the prctl flag is PR_THP_DISABLE_EXCEPT_ADVISED? >>> >>> THPs are disabled for these regions, so it's at least consistent with the "disable all", but ... >>> >>>> >>>> This will be checked in multiple places in madvise_collapse: thp_vma_allowable_order, >>>> hugepage_vma_revalidate which calls thp_vma_allowable_order and hpage_collapse_scan_pmd >>>> which also ends up calling hugepage_vma_revalidate. >>>>> A hacky way would be to save and overwrite vma->vm_flags with VM_HUGEPAGE at the start of madvise_collapse >>>> if VM_NOHUGEPAGE is not set, and reset vma->vm_flags to its original value at the end of madvise_collapse >>>> (Not something I am recommending, just throwing it out there). >>> >>> Gah. >>> >>>> >>>> Another possibility is to pass the fact that you are in madvise_collapse to these functions >>>> as an argument, this might look ugly, although maybe not as ugly as hugepage_vma_revalidate >>>> already has collapse control arg, so just need to take care of thp_vma_allowable_orders. >>> >>> Likely this. >>> >>>> >>>> Any preference or better suggestions? >>> >>> What you are asking for is not MMF_DISABLE_THP_EXCEPT_ADVISED as I planned it, but MMF_DISABLE_THP_EXCEPT_ADVISED_OR_MADV_COLLAPSE. >>> >>> Now, one could consider MADV_COLLAPSE an "advise". (I am not opposed to that change) >>> >> >> lol yeah I always think of MADV_COLLAPSE as an extreme version of MADV_HUGE (more of a demand >> than an advice :)), eventhough its not persistant. >> Which is why I think might be unexpected if MADV_HUGE gives hugepages but MADV_COLLAPSE doesn't >> (But could just be my opinion). >> >>> Indeed, the right way might be telling vma_thp_disabled() whether we are in collapse. >>> >>> Can you try implementing that on top of my patch to see how it looks? >>> >> >> My reasoning is that a process that is running with system policy always but with >> PR_THP_DISABLE_EXCEPT_ADVISED gets THPs in exactly the same behaviour as a process that is running >> with system policy madvise. This will help us achieve (3) that you mentioned in the >> commit message: >> (3) Switch from THP=madvise to THP=always, but keep the old behavior >> (THP only when advised) for selected workloads. >> >> >> I have written quite a few selftests now for prctl SET_THP_DISABLE, both with and without >> PR_THP_DISABLE_EXCEPT_ADVISED set incorporating your feedback on it. I have all of them passing >> with the below diff. The diff is slightly ugly, but very simple and hopefully acceptable. If it >> looks good, I can send a series with everything. Probably make the below diff as a separate patch >> on top of this patch as its mostly adding an extra arg to functions and would keep the review easier? > > Yes, we should do it as a separate patch, makes our life easier, because that requires more work. > > We require a cleanup first, the boolean parameter for __thp_vma_allowable_orders() is no good. > > I just pushed something untested to my branch (slightly adjusted patch#1 + 2 more patches), can you have a look at that? (untested ... :) ) > Thanks for this! I tested it and its good, have sent it for review.
