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Message-ID: <856f4317-43d9-41bb-8096-1eef69c86d3b@arm.com>
Date: Thu, 7 Dec 2023 11:44:03 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>,
Yin Fengwei <fengwei.yin@...el.com>,
Yu Zhao <yuzhao@...gle.com>,
Catalin Marinas <catalin.marinas@....com>,
Anshuman Khandual <anshuman.khandual@....com>,
Yang Shi <shy828301@...il.com>,
"Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
Luis Chamberlain <mcgrof@...nel.org>,
Itaru Kitayama <itaru.kitayama@...il.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
John Hubbard <jhubbard@...dia.com>,
David Rientjes <rientjes@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Hugh Dickins <hughd@...gle.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Barry Song <21cnbao@...il.com>,
Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 03/10] mm: thp: Introduce multi-size THP sysfs
interface
On 07/12/2023 11:25, David Hildenbrand wrote:
> On 07.12.23 12:22, Ryan Roberts wrote:
>> On 07/12/2023 11:13, David Hildenbrand wrote:
>>>>>
>>>>>> +
>>>>>> if (!vma->vm_mm) /* vdso */
>>>>>> - return false;
>>>>>> + return 0;
>>>>>> /*
>>>>>> * Explicitly disabled through madvise or prctl, or some
>>>>>> @@ -88,16 +141,16 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>>>>>> unsigned long vm_flags,
>>>>>> * */
>>>>>> if ((vm_flags & VM_NOHUGEPAGE) ||
>>>>>> test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>>>>>> - return false;
>>>>>> + return 0;
>>>>>> /*
>>>>>> * If the hardware/firmware marked hugepage support disabled.
>>>>>> */
>>>>>> if (transparent_hugepage_flags & (1 <<
>>>>>> TRANSPARENT_HUGEPAGE_UNSUPPORTED))
>>>>>> - return false;
>>>>>> + return 0;
>>>>>> /* khugepaged doesn't collapse DAX vma, but page fault is fine. */
>>>>>> if (vma_is_dax(vma))
>>>>>> - return in_pf;
>>>>>> + return in_pf ? orders : 0;
>>>>>> /*
>>>>>> * khugepaged special VMA and hugetlb VMA.
>>>>>> @@ -105,17 +158,29 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>>>>>> unsigned long vm_flags,
>>>>>> * VM_MIXEDMAP set.
>>>>>> */
>>>>>> if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))
>>>>>> - return false;
>>>>>> + return 0;
>>>>>> /*
>>>>>> - * Check alignment for file vma and size for both file and anon vma.
>>>>>> + * Check alignment for file vma and size for both file and anon vma by
>>>>>> + * filtering out the unsuitable orders.
>>>>>> *
>>>>>> * Skip the check for page fault. Huge fault does the check in fault
>>>>>> - * handlers. And this check is not suitable for huge PUD fault.
>>>>>> + * handlers.
>>>>>> */
>>>>>> - if (!in_pf &&
>>>>>> - !transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
>>>>>> - return false;
>>>>>> + if (!in_pf) {
>>>>>> + int order = first_order(orders);
>>>>>> + unsigned long addr;
>>>>>> +
>>>>>> + while (orders) {
>>>>>> + addr = vma->vm_end - (PAGE_SIZE << order);
>>>>>> + if (thp_vma_suitable_orders(vma, addr, BIT(order)))
>>>>>> + break;
>>>>>
>>>>> Comment: you'd want a "thp_vma_suitable_order" helper here. But maybe the
>>>>> compiler is smart enough to optimize the loop and everyything else out.
>>>>
>>>> I'm happy to refactor so that thp_vma_suitable_order() is the basic primitive,
>>>> then make thp_vma_suitable_orders() a loop that calls thp_vma_suitable_order()
>>>> (that's basically how it is laid out already, just all in one function). Is
>>>> that
>>>> what you are requesting?
>>>
>>> You got the spirit, yes.
>>>
>>>>>
>>>>> [...]
>>>>>
>>>>>> +
>>>>>> +static ssize_t thpsize_enabled_store(struct kobject *kobj,
>>>>>> + struct kobj_attribute *attr,
>>>>>> + const char *buf, size_t count)
>>>>>> +{
>>>>>> + int order = to_thpsize(kobj)->order;
>>>>>> + ssize_t ret = count;
>>>>>> +
>>>>>> + if (sysfs_streq(buf, "always")) {
>>>>>> + set_bit(order, &huge_anon_orders_always);
>>>>>> + clear_bit(order, &huge_anon_orders_inherit);
>>>>>> + clear_bit(order, &huge_anon_orders_madvise);
>>>>>> + } else if (sysfs_streq(buf, "inherit")) {
>>>>>> + set_bit(order, &huge_anon_orders_inherit);
>>>>>> + clear_bit(order, &huge_anon_orders_always);
>>>>>> + clear_bit(order, &huge_anon_orders_madvise);
>>>>>> + } else if (sysfs_streq(buf, "madvise")) {
>>>>>> + set_bit(order, &huge_anon_orders_madvise);
>>>>>> + clear_bit(order, &huge_anon_orders_always);
>>>>>> + clear_bit(order, &huge_anon_orders_inherit);
>>>>>> + } else if (sysfs_streq(buf, "never")) {
>>>>>> + clear_bit(order, &huge_anon_orders_always);
>>>>>> + clear_bit(order, &huge_anon_orders_inherit);
>>>>>> + clear_bit(order, &huge_anon_orders_madvise);
>>>>>
>>>>> Note: I was wondering for a second if some concurrent cames could lead to an
>>>>> inconsistent state. I think in the worst case we'll simply end up with "never"
>>>>> on races.
>>>>
>>>> You mean if different threads try to write different values to this file
>>>> concurrently? Or if there is a concurrent fault that tries to read the flags
>>>> while they are being modified?
>>>
>>> I thought about what you said first, but what you said last might also apply. As
>>> long as "nothing breaks", all good.
>>>
>>>>
>>>> I thought about this for a long time too and wasn't sure what was best. The
>>>> existing global enabled store impl clears the bits first then sets the bit.
>>>> With
>>>> this approach you can end up with multiple bits set if there is a race to set
>>>> diffierent values, and you can end up with a faulting thread seeing never if it
>>>> reads the bits after they have been cleared but before setting them.
>>>
>>> Right, but user space is playing stupid games and can win stupid prices. As long
>>> as nothing breaks, we're good.
>>>
>>>>
>>>> I decided to set the new bit before clearing the old bits, which is
>>>> different; A
>>>> racing fault will never see "never" but as you say, a race to set the file
>>>> could
>>>> result in "never" being set.
>>>>
>>>> On reflection, it's probably best to set the bit *last* like the global control
>>>> does?
>>>
>>> Probably might just slap a simple spinlock in there, so at least the writer side
>>> is completely serialized. Then you can just set the bit last. It's unlikely that
>>> readers will actually run into issues, and if they ever would, we could use some
>>> rcu magic to let them read a consistent state.
>>
>> I'd prefer to leave it as it is now; clear first, set last without any explicit
>> serialization. I've convinced myself that nothing breaks and its the same
>> pattern used by the global control so its consistent. Unless you're insisting on
>> the spin lock?
>
> No, not at all. But it would certainly remove any possible concerns :)
OK fine, you win :). I'll add a spin lock on the writer side.
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