Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 12 Feb 2021 15:40:28 -0500
From:   Peter Xu <peterx@redhat.com>
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Axel Rasmussen <axelrasmussen@google.com>
Cc:     Axel Rasmussen <axelrasmussen@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Chinwen Chang <chinwen.chang@mediatek.com>,
        Huang Ying <ying.huang@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Lokesh Gidra <lokeshgidra@google.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>,
        Michel Lespinasse <walken@google.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@gmail.com>, Shaohua Li <shli@fb.com>,
        Shawn Anastasio <shawn@anastas.io>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Price <steven.price@arm.com>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        Adam Ruprecht <ruprecht@google.com>,
        Cannon Matthews <cannonmatthews@google.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Oliver Upton <oupton@google.com>
Subject: Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing
 when uffd enabled
Message-ID: <20210212204028.GC3171@xz-x1>
References: <20210210212200.1097784-1-axelrasmussen@google.com>
 <20210210212200.1097784-3-axelrasmussen@google.com>
 <0a991b83-18f8-cd76-46c0-4e0dcd5c87a7@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <0a991b83-18f8-cd76-46c0-4e0dcd5c87a7@oracle.com>
Precedence: bulk

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