From: William Kucharski <william.kucharski@oracle.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 14/15] mm: Align THP mappings for non-DAX
Date: Tue, 1 Oct 2019 10:08:30 -0600 [thread overview]
Message-ID: <85868A27-CA43-4AF5-B8A0-2D037C2207FD@oracle.com> (raw)
In-Reply-To: <20191001142018.wpordswdkadac6kt@box>
> On Oct 1, 2019, at 8:20 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Tue, Oct 01, 2019 at 06:18:28AM -0600, William Kucharski wrote:
>>
>>
>> On 10/1/19 5:32 AM, Kirill A. Shutemov wrote:
>>> On Tue, Oct 01, 2019 at 05:21:26AM -0600, William Kucharski wrote:
>>>>
>>>>
>>>>> On Oct 1, 2019, at 4:45 AM, Kirill A. Shutemov <kirill@shutemov.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?
>>>
>>> We need to know by the time if THP is allowed for this
>>> file/VMA/process/whatever. Meaning that we do not give up ASLR entropy for
>>> nothing.
>>>
>>> For instance, if THP is disabled globally, there is no reason to align the
>>> VMA to the THP requirements.
>>
>> I understand, but this code is in thp_get_unmapped_area(), which is only called
>> if THP is configured and the VMA can support it.
>>
>> I don't see it in Matthew's patchset, so I'm not sure if it was inadvertently
>> missed in his merge or if he has other ideas for how it would eventually be
>> called, but in my last patch revision the code calling it in do_mmap()
>> looked like this:
>>
>> #ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
>> /*
>> * If THP is enabled, it's a read-only executable that is
>> * MAP_PRIVATE mapped, the length is larger than a PMD page
>> * and either it's not a MAP_FIXED mapping or the passed address is
>> * properly aligned for a PMD page, attempt to get an appropriate
>> * address at which to map a PMD-sized THP page, otherwise call the
>> * normal routine.
>> */
>> if ((prot & PROT_READ) && (prot & PROT_EXEC) &&
>> (!(prot & PROT_WRITE)) && (flags & MAP_PRIVATE) &&
>> (!(flags & MAP_FIXED)) && len >= HPAGE_PMD_SIZE) {
>
> len and MAP_FIXED is already handled by thp_get_unmapped_area().
>
> if (prot & (PROT_READ|PROT_WRITE|PROT_READ) == (PROT_READ|PROT_EXEC) &&
> (flags & MAP_PRIVATE)) {
It is, but I wanted to avoid even calling it if conditions weren't right.
Checking twice is non-optimal but I didn't want to alter the existing use of
the routine for anon THP.
>
>
>> addr = thp_get_unmapped_area(file, addr, len, pgoff, flags);
>>
>> if (addr && (!(addr & ~HPAGE_PMD_MASK))) {
>
> This check is broken.
>
> For instance, if pgoff is one, (addr & ~HPAGE_PMD_MASK) has to be equal to
> PAGE_SIZE to have chance to get a huge page in the mapping.
>
If the address isn't PMD-aligned, we will never be able to map it with a THP
anyway.
The current code is designed to only map a THP if the VMA allows for it and
it can map the entire THP starting at an aligned address.
You can't map a THP at the PMD level at an address that isn't PMD aligned.
Perhaps I'm missing a use case here.
>> /*
>> * If we got a suitable THP mapping address, shut off
>> * VM_MAYWRITE for the region, since it's never what
>> * we would want.
>> */
>> vm_maywrite = 0;
>
> Wouldn't it break uprobe, for instance?
I'm not sure; does uprobe allow COW to insert the probe even for mappings
explicitly marked read-only?
Thanks,
Bill
next prev parent reply other threads:[~2019-10-01 16:08 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-25 0:51 [RFC 00/15] Large pages in the page-cache Matthew Wilcox
2019-09-25 0:52 ` [PATCH 01/15] mm: Use vm_fault error code directly Matthew Wilcox
2019-09-26 13:55 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 02/15] fs: Introduce i_blocks_per_page Matthew Wilcox
2019-09-25 8:36 ` Dave Chinner
2019-10-04 19:28 ` Matthew Wilcox
2019-10-08 3:53 ` Dave Chinner
2019-09-25 0:52 ` [PATCH 03/15] mm: Add file_offset_of_ helpers Matthew Wilcox
2019-09-26 14:02 ` Kirill A. Shutemov
2019-10-04 19:39 ` Matthew Wilcox
2019-09-25 0:52 ` [PATCH 04/15] iomap: Support large pages Matthew Wilcox
2019-09-25 0:52 ` [PATCH 05/15] xfs: " Matthew Wilcox
2019-09-25 0:52 ` [PATCH 06/15] xfs: Pass a page to xfs_finish_page_writeback Matthew Wilcox
2019-09-25 0:52 ` [PATCH 07/15] mm: Make prep_transhuge_page tail-callable Matthew Wilcox
2019-09-26 14:13 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 08/15] mm: Add __page_cache_alloc_order Matthew Wilcox
2019-09-26 14:15 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 09/15] mm: Allow large pages to be added to the page cache Matthew Wilcox
2019-09-26 14:22 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 10/15] mm: Allow find_get_page to be used for large pages Matthew Wilcox
2019-10-01 10:32 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 11/15] mm: Remove hpage_nr_pages Matthew Wilcox
2019-10-01 10:35 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 12/15] mm: Support removing arbitrary sized pages from mapping Matthew Wilcox
2019-10-01 10:39 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 13/15] mm: Add a huge page fault handler for files Matthew Wilcox
2019-10-01 10:42 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 14/15] mm: Align THP mappings for non-DAX Matthew Wilcox
2019-10-01 10:45 ` Kirill A. Shutemov
2019-10-01 11:21 ` William Kucharski
2019-10-01 11:32 ` Kirill A. Shutemov
2019-10-01 12:18 ` William Kucharski
2019-10-01 14:20 ` Kirill A. Shutemov
2019-10-01 16:08 ` William Kucharski [this message]
2019-10-02 0:15 ` Kirill A. Shutemov
2019-09-25 0:52 ` [PATCH 15/15] xfs: Use filemap_huge_fault Matthew Wilcox
2019-10-02 13:07 ` [PATCH 03/15] mm: Add file_offset_of_ helpers Hillf Danton
2019-10-04 19:33 ` Matthew Wilcox
2019-10-02 13:32 ` [PATCH 04/15] iomap: Support large pages Hillf Danton
2019-10-04 19:34 ` Matthew Wilcox
2019-10-03 4:08 ` [PATCH 06/15] xfs: Pass a page to xfs_finish_page_writeback Hillf Danton
2019-10-04 19:35 ` Matthew Wilcox
2019-10-03 5:08 ` [PATCH 11/15] mm: Remove hpage_nr_pages Hillf Danton
2019-10-04 19:36 ` Matthew Wilcox
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