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Subject: Re: [PATCH v3 2/2] mm,thp: Add experimental config option
 RO_EXEC_FILEMAP_HUGE_FAULT_THP
From:   William Kucharski <william.kucharski@oracle.com>
In-Reply-To: <20190806111210.7xpmjsd4hq54vuml@box>
Date:   Wed, 7 Aug 2019 10:12:35 -0600
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Song Liu <songliubraving@fb.com>,
        Bob Kasten <robert.a.kasten@intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Chad Mynhier <chad.mynhier@oracle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Johannes Weiner <jweiner@fb.com>,
        Matthew Wilcox <willy@infradead.org>
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To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Sender: linux-kernel-owner@vger.kernel.org
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> On Aug 6, 2019, at 5:12 AM, Kirill A. Shutemov <kirill@shutemov.name> =
wrote:
>=20
> IIUC, you are missing ->vm_pgoff from the picture. The newly allocated
> page must land into page cache aligned on HPAGE_PMD_NR boundary. In =
other
> word you cannout have huge page with ->index, let say, 1.
>=20
> VMA is only suitable for at least one file-THP page if:
>=20
> - (vma->vm_start >> PAGE_SHIFT) % (HPAGE_PMD_NR - 1) is equal to
>    vma->vm_pgoff % (HPAGE_PMD_NR - 1)
>=20
>    This guarantees right alignment in the backing page cache.
>=20
> - *and* vma->vm_end - round_up(vma->vm_start, HPAGE_PMD_SIZE) is equal =
or
>   greater than HPAGE_PMD_SIZE.
>=20
> Does it make sense?

It makes sense, but what I am thinking was say a vma->vm_start of =
0x1ff000
and vma->vm_end of 0x400000.

Assuming x86, that can be mapped with a PAGESIZE page at 0x1ff000 then a
PMD page mapping 0x200000 - 0x400000.

That doesn't mean a vma IS or COULD ever be configured that way, so you =
are
correct with your comment, and I will change my check accordingly.

>> In the current code, it's assumed it is not exposed, because a single =
read
>> of a large page that does no readahead before the page is inserted =
into the
>> cache means there are no external users of the page.
>=20
> You've exposed the page to the filesystem once you call ->readpage().
> It *may* track the page somehow after the call.

OK, thanks again.

I'll try to have a V4 available with these changes soon.