linux-kernel - Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

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Date:   Fri, 12 Feb 2021 15:40:28 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing
 when uffd enabled

On Thu, Feb 11, 2021 at 04:19:55PM -0800, Mike Kravetz wrote:
> want_pmd_share() is currently just a check for CONFIG_ARCH_WANT_HUGE_PMD_SHARE.
> How about leaving that mostly as is, and adding the new vma checks to
> vma_shareable().  vma_shareable() would then be something like:
> 
> 	if (!(vma->vm_flags & VM_MAYSHARE))
> 		return false;
> #ifdef CONFIG_USERFAULTFD
> 	if (uffd_disable_huge_pmd_share(vma)
> 		return false;
> #endif
> #ifdef /* XXX */
> 	/* add other checks for things like uffd wp and soft dirty here */
> #endif /* XXX */
> 
> 	if (range_in_vma(vma, base, end)
> 		return true;
> 	return false;
> 
> Of course, this would require we leave the call to vma_shareable() at the
> beginning of huge_pmd_share.  It also means that we are always making a
> function call into huge_pmd_share to determine if sharing is possible.
> That is not any different than today.  If we do not want to make that extra
> function call, then I would suggest putting all that code in want_pmd_share.
> It just seems that all the vma checks for sharing should be in one place
> if possible.

I don't worry a lot on that since we've already got huge_pte_alloc() which
takes care of huge pmd sharing case, so I don't expect e.g. even most hugetlb
developers to use want_pmd_share() at all, because huge_pte_alloc() will be the
one that frequently got called.

But yeah we can definitely put the check logic into huge_pmd_share() too.
Looking at above code it looks still worth a helper like want_pmd_share() or
with some other name.  Then... instead of making this complicated, how about I
mostly keep this patch but move want_pmd_share() call into huge_pmd_share()
instead?

Btw, Axel, it seems there will still be some respins on the pmd sharing
patches.  Since it turns out it'll be shared by multiple tasks now, do you mind
I pick those out and send them separately?  Then we can consolidate this part
to move on with either the rest of the tasks we've got on hand.

Thanks,

-- 
Peter Xu