linux-kernel - Re: [PATCH v3 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT

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Date:   Wed, 7 Aug 2019 10:12:35 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Bob Kasten <robert.a.kasten@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Chad Mynhier <chad.mynhier@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <jweiner@...com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 2/2] mm,thp: Add experimental config option
 RO_EXEC_FILEMAP_HUGE_FAULT_THP



> On Aug 6, 2019, at 5:12 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> 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.
> 
> VMA is only suitable for at least one file-THP page if:
> 
> - (vma->vm_start >> PAGE_SHIFT) % (HPAGE_PMD_NR - 1) is equal to
>    vma->vm_pgoff % (HPAGE_PMD_NR - 1)
> 
>    This guarantees right alignment in the backing page cache.
> 
> - *and* vma->vm_end - round_up(vma->vm_start, HPAGE_PMD_SIZE) is equal or
>   greater than HPAGE_PMD_SIZE.
> 
> 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.
> 
> 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.