linux-kernel - Re: Linux 4.18-rc7

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Date:   Wed, 1 Aug 2018 23:58:49 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tony Luck <tony.luck@...el.com>,
        Amit Pundir <amit.pundir@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        youling 257 <youling257@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Colin Cross <ccross@...gle.com>
Subject: Re: Linux 4.18-rc7

On Wed, Aug 01, 2018 at 11:31:52AM -0700, Hugh Dickins wrote:
> On Wed, 1 Aug 2018, Linus Torvalds wrote:
> > 
> > Anyway, the upshot of all this is that I think I know what the ia64
> > problem was, and John sent the patch for the ashmem case, and I'm
> > going to hold off reverting that vma_is_anonymous() false-positives
> > commit after all.
> 
> I'd better send deletion of zap_pmd_range()'s VM_BUG_ON_VMA(): below
> (but I've no proprietorial interest, if you prefer to do your own).

Agreed.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

> John's patch is good, and originally I thought it was safe from that
> VM_BUG_ON_VMA(), because the /dev/ashmem fd exposed to the user is
> disconnected from the vm_file in the vma, and madvise(,,MADV_REMOVE)
> insists on VM_SHARED. But afterwards read John's earlier mail,
> drawing attention to the vfs_fallocate() in there: I may be wrong,
> and I don't know if Android has THP in the config anyway, but it looks
> to me like an unmap_mapping_range() from ashmem's vfs_fallocate()
> could hit precisely the VM_BUG_ON_VMA(), once it's vma_is_anonymous().
> 
> (I'm not familiar with ashmem, and I certainly don't understand the
> role of MAP_PRIVATE ashmem mappings - hole-punch's zap_pte_range()
> should end up leaving any anon pages in place; but the presence of
> the BUG is requiring us all to understand too much too quickly.)

Hugh, do you see any reason why ashmem shouldn't have vm_ops ==
shmem_vm_ops?

I don't understand ashmem, but I feel uncomfortable that we have this
sneaky way to create an anonymous VMA. It feels wrong to me.

-- 
 Kirill A. Shutemov