Re: [PATCH v5 4/6] mm: shmem: add mTHP support for anonymous shmem

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/7/4 01:25, Ryan Roberts wrote:
> On 11/06/2024 11:11, Baolin Wang wrote:
>> Commit 19eaf44954df adds multi-size THP (mTHP) for anonymous pages, that
>> can allow THP to be configured through the sysfs interface located at
>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
>>
>> However, the anonymous shmem will ignore the anonymous mTHP rule
>> configured through the sysfs interface, and can only use the PMD-mapped
>> THP, that is not reasonable. Users expect to apply the mTHP rule for
>> all anonymous pages, including the anonymous shmem, in order to enjoy
>> the benefits of mTHP. For example, lower latency than PMD-mapped THP,
>> smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture
>> to reduce TLB miss etc. In addition, the mTHP interfaces can be extended
>> to support all shmem/tmpfs scenarios in the future, especially for the
>> shmem mmap() case.
>>
>> The primary strategy is similar to supporting anonymous mTHP. Introduce
>> a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled',
>> which can have almost the same values as the top-level
>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new
>> additional "inherit" option and dropping the testing options 'force' and
>> 'deny'. By default all sizes will be set to "never" except PMD size,
>> which is set to "inherit". This ensures backward compatibility with the
>> anonymous shmem enabled of the top level, meanwhile also allows independent
>> control of anonymous shmem enabled for each mTHP.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> [...]
> 
> Hi Baolin,
> 
> I'm currently trying to fix a bug where khugepaged is not started if only shmem
> is enabled for THP. See discussion at [1]. It's been broken like this forever.
> 
> Assuming anon and shmem THP are initially both disabled:
> 
> # echo never > /sys/kernel/mm/transparent_hugepage/shmem_enabled
> # echo never > /sys/kernel/mm/transparent_hugepage/enabled
> <khugepaged is stopped here>
> 
> Then shemem THP is enabled:
> 
> # echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled
> <khugepaged is not started, this is a bug>

Thanks Ryan. Yes, this is a real problem.

> As part of investigating the fix, I stumbled upon this patch, which I remember
> reviewing an early version of but I've been out for a while and missed the
> latter versions. See below for comments and questions; the answers to which will
> help me figure out how to fix the above...
> 
> [1] https://lore.kernel.org/linux-mm/20240702144617.2291480-1-ryan.roberts@arm.com/
> 
> 
>>   
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static unsigned long shmem_allowable_huge_orders(struct inode *inode,
>> +				struct vm_area_struct *vma, pgoff_t index,
>> +				bool global_huge)
>> +{
>> +	unsigned long mask = READ_ONCE(huge_shmem_orders_always);
>> +	unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
>> +	unsigned long vm_flags = vma->vm_flags;
>> +	/*
>> +	 * Check all the (large) orders below HPAGE_PMD_ORDER + 1 that
>> +	 * are enabled for this vma.
>> +	 */
>> +	unsigned long orders = BIT(PMD_ORDER + 1) - 1;
>> +	loff_t i_size;
>> +	int order;
>> +
>> +	if ((vm_flags & VM_NOHUGEPAGE) ||
>> +	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> +		return 0;
>> +
>> +	/* If the hardware/firmware marked hugepage support disabled. */
>> +	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_UNSUPPORTED))
>> +		return 0;
>> +
>> +	/*
>> +	 * Following the 'deny' semantics of the top level, force the huge
>> +	 * option off from all mounts.
>> +	 */
>> +	if (shmem_huge == SHMEM_HUGE_DENY)
>> +		return 0;
> 
> I don't quite get this, I don't think its the desirable behaviour. This is
> saying that if the top-level shemem_enabled control is set to 'deny', then all
> mTHP sizes, regardless of their control's setting are disabled?
> 
> The anon controls don't work like that; you can set the top-level control to
> anything you like, but its only consumed if the per-size controls are inheriting it.

IMO, 'deny' option is not similar like 'never' option.

> 
> So for the deny case, wouldn't it be better to allow that as an option on all
> the per-size controls (and implicitly let it be inherrited from the top-level)?

 From 'deny' option's semantics:
"disables huge on shm_mnt and all mounts, for emergency use;"

It is usually used for testing to shut down all huge pages from the old 
ages. Moreover, mTHP interfaces will be used as a huge order filter to 
support tmpfs, so I think it will make life easier to disable all huge 
orders for testing or emergency use, as well as to maintain the original 
semantics.

>> +
>> +	/*
>> +	 * Only allow inherit orders if the top-level value is 'force', which
>> +	 * means non-PMD sized THP can not override 'huge' mount option now.
>> +	 */
>> +	if (shmem_huge == SHMEM_HUGE_FORCE)
>> +		return READ_ONCE(huge_shmem_orders_inherit);
> 
> I vaguely recall that we originally discussed that trying to set 'force' on the
> top level control while any per-size controls were set to 'inherit' would be an
> error, and trying to set 'force' on any per-size control except the PMD-size
> would be an error?

Right.

> I don't really understand this logic. Shouldn't we just be looking at the
> per-size control settings (or the top-level control as a proxy for every
> per-size control that has 'inherit' set)?

‘force’ will apply the huge orders for anon shmem and tmpfs, so now we 
only allow pmd-mapped THP to be forced. We should not look at per-size 
control settings for tmpfs now (mTHP for tmpfs will be discussed in future).

> 
> Then for tmpfs, which doesn't support non-PMD-sizes yet, we just always use the
> PMD-size control for decisions.
> 
> I'm also really struggling with the concept of shmem_is_huge() existing along
> side shmem_allowable_huge_orders(). Surely this needs to all be refactored into
> shmem_allowable_huge_orders()?

I understood. But now they serve different purposes: shmem_is_huge() 
will be used to check the huge orders for the top level, for *tmpfs* and 
anon shmem; whereas shmem_allowable_huge_orders() will only be used to 
check the per-size huge orders for anon shmem (excluding tmpfs now). 
However, as I plan to add mTHP support for tmpfs, I think we can perform 
some cleanups.

>> +	/* Allow mTHP that will be fully within i_size. */
>> +	order = highest_order(within_size_orders);
>> +	while (within_size_orders) {
>> +		index = round_up(index + 1, order);
>> +		i_size = round_up(i_size_read(inode), PAGE_SIZE);
>> +		if (i_size >> PAGE_SHIFT >= index) {
>> +			mask |= within_size_orders;
>> +			break;
>> +		}
>> +
>> +		order = next_order(&within_size_orders, order);
>> +	}
>> +
>> +	if (vm_flags & VM_HUGEPAGE)
>> +		mask |= READ_ONCE(huge_shmem_orders_madvise);
>> +
>> +	if (global_huge)
> 
> Perhaps I've misunderstood global_huge, but I think its just the return value
> from shmem_is_huge()? But you're also using shmem_huge directly in this

Yes.

> function. I find it all rather confusing.

I think I have explained why need these logics as above. Since mTHP 
support for shmem has just started (tmpfs is still in progress). I will 
make it more clear in the following patches.

> Sorry if all this was discussed and agreed in my absence! But I think this needs
> to be sorted in order to do the bug fix for khugepaged properly.

No worries. Thanks for your input.