linux-kernel - Re: [PATCH 14/15] mm: Align THP mappings for non-DAX

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Date:   Tue, 1 Oct 2019 05:21:26 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/15] mm: Align THP mappings for non-DAX



> On Oct 1, 2019, at 4:45 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> On Tue, Sep 24, 2019 at 05:52:13PM -0700, Matthew Wilcox wrote:
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index cbe7d0619439..670a1780bd2f 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -563,8 +563,6 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>> 
>> 	if (addr)
>> 		goto out;
>> -	if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
>> -		goto out;
>> 
>> 	addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
>> 	if (addr)
> 
> I think you reducing ASLR without any real indication that THP is relevant
> for the VMA. We need to know if any huge page allocation will be
> *attempted* for the VMA or the file.

Without a properly aligned address the code will never even attempt allocating
a THP.

I don't think rounding an address to one that would be properly aligned to map
to a THP if possible is all that detrimental to ASLR and without the ability to
pick an aligned address it's rather unlikely anyone would ever map anything to
a THP unless they explicitly designate an address with MAP_FIXED.

If you do object to the slight reduction of the ASLR address space, what
alternative would you prefer to see?

    -- Bill