Re: [PATCH v2 3/5] mm: shmem: add large folio support for tmpfs

[David Hildenbrand <david@redhat.com>]

On 12.11.24 08:45, Baolin Wang wrote:
> Add large folio support for tmpfs write and fallocate paths matching the
> same high order preference mechanism used in the iomap buffered IO path
> as used in __filemap_get_folio().
> 
> Add shmem_mapping_size_orders() to get a hint for the orders of the folio
> based on the file size which takes care of the mapping requirements.
> 
> Traditionally, tmpfs only supported PMD-sized huge folios. However nowadays
> with other file systems supporting any sized large folios, and extending
> anonymous to support mTHP, we should not restrict tmpfs to allocating only
> PMD-sized huge folios, making it more special. Instead, we should allow
> tmpfs can allocate any sized large folios.
> 
> Considering that tmpfs already has the 'huge=' option to control the huge
> folios allocation, we can extend the 'huge=' option to allow any sized huge
> folios. The semantics of the 'huge=' mount option are:
> 
> huge=never: no any sized huge folios
> huge=always: any sized huge folios
> huge=within_size: like 'always' but respect the i_size
> huge=advise: like 'always' if requested with fadvise()/madvise()
> 
> Note: for tmpfs mmap() faults, due to the lack of a write size hint, still
> allocate the PMD-sized huge folios if huge=always/within_size/advise is set.
> 
> Moreover, the 'deny' and 'force' testing options controlled by
> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', still retain the same
> semantics. The 'deny' can disable any sized large folios for tmpfs, while
> the 'force' can enable PMD sized large folios for tmpfs.
> 
> Co-developed-by: Daniel Gomez <da.gomez@samsung.com>
> Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/shmem.c | 91 +++++++++++++++++++++++++++++++++++++++++++++---------
>   1 file changed, 77 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 86b2e417dc6f..a3203cf8860f 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -549,10 +549,50 @@ static bool shmem_confirm_swap(struct address_space *mapping,
>   
>   static int shmem_huge __read_mostly = SHMEM_HUGE_NEVER;
>   
> +/**
> + * shmem_mapping_size_orders - Get allowable folio orders for the given file size.
> + * @mapping: Target address_space.
> + * @index: The page index.
> + * @write_end: end of a write, could extend inode size.
> + *
> + * This returns huge orders for folios (when supported) based on the file size
> + * which the mapping currently allows at the given index. The index is relevant
> + * due to alignment considerations the mapping might have. The returned order
> + * may be less than the size passed.
> + *
> + * Return: The orders.
> + */
> +static inline unsigned int
> +shmem_mapping_size_orders(struct address_space *mapping, pgoff_t index, loff_t write_end)
> +{
> +	unsigned int order;
> +	size_t size;
> +
> +	if (!mapping_large_folio_support(mapping) || !write_end)
> +		return 0;
> +
> +	/* Calculate the write size based on the write_end */
> +	size = write_end - (index << PAGE_SHIFT);
> +	order = filemap_get_order(size);
> +	if (!order)
> +		return 0;
> +
> +	/* If we're not aligned, allocate a smaller folio */
> +	if (index & ((1UL << order) - 1))
> +		order = __ffs(index);
> +
> +	order = min_t(size_t, order, MAX_PAGECACHE_ORDER);
> +	return order > 0 ? BIT(order + 1) - 1 : 0;
> +}
> +
>   static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
>   					      loff_t write_end, bool shmem_huge_force,
> +					      struct vm_area_struct *vma,
>   					      unsigned long vm_flags)
>   {
> +	unsigned long within_size_orders;
> +	unsigned int order;
> +	pgoff_t aligned_index;
>   	loff_t i_size;
>   
>   	if (HPAGE_PMD_ORDER > MAX_PAGECACHE_ORDER)

We can allow all orders up to MAX_PAGECACHE_ORDER, 
shmem_mapping_size_orders() handles it properly.

So maybe we should drop this condition and use instead below where we have

return BIT(HPAGE_PMD_ORDER);

instead something like.

return HPAGE_PMD_ORDER > MAX_PAGECACHE_ORDER ? 0 : BIT(HPAGE_PMD_ORDER);

Ideally, factoring it out somehow


int maybe_pmd_order = HPAGE_PMD_ORDER > MAX_PAGECACHE_ORDER ? 0 : 
BIT(HPAGE_PMD_ORDER);

...

return maybe_pmd_order;

> @@ -564,15 +604,41 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
>   	if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
>   		return BIT(HPAGE_PMD_ORDER);

Why not force-enable all orders (of course, respecting 
MAX_PAGECACHE_ORDER and possibly VMA)?

>   
> +	/*
> +	 * The huge order allocation for anon shmem is controlled through
> +	 * the mTHP interface, so we still use PMD-sized huge order to
> +	 * check whether global control is enabled.
> +	 *
> +	 * For tmpfs mmap()'s huge order, we still use PMD-sized order to
> +	 * allocate huge pages due to lack of a write size hint.
> +	 *
> +	 * Otherwise, tmpfs will allow getting a highest order hint based on
> +	 * the size of write and fallocate paths, then will try each allowable
> +	 * huge orders.
> +	 */
>   	switch (SHMEM_SB(inode->i_sb)->huge) {
>   	case SHMEM_HUGE_ALWAYS:
> -		return BIT(HPAGE_PMD_ORDER);
> -	case SHMEM_HUGE_WITHIN_SIZE:
> -		index = round_up(index + 1, HPAGE_PMD_NR);
> -		i_size = max(write_end, i_size_read(inode));
> -		i_size = round_up(i_size, PAGE_SIZE);
> -		if (i_size >> PAGE_SHIFT >= index)
> +		if (vma)
>   			return BIT(HPAGE_PMD_ORDER);
> +
> +		return shmem_mapping_size_orders(inode->i_mapping, index, write_end);
> +	case SHMEM_HUGE_WITHIN_SIZE:
> +		if (vma)
> +			within_size_orders = BIT(HPAGE_PMD_ORDER);
> +		else
> +			within_size_orders = shmem_mapping_size_orders(inode->i_mapping,
> +								       index, write_end);
> +
> +		order = highest_order(within_size_orders);
> +		while (within_size_orders) {
> +			aligned_index = round_up(index + 1, 1 << order);
> +			i_size = max(write_end, i_size_read(inode));
> +			i_size = round_up(i_size, PAGE_SIZE);
> +			if (i_size >> PAGE_SHIFT >= aligned_index)
> +				return within_size_orders;
> +
> +			order = next_order(&within_size_orders, order);
> +		}
>   		fallthrough;
>   	case SHMEM_HUGE_ADVISE:
>   		if (vm_flags & VM_HUGEPAGE)

I think the point here is that "write" -> no VMA -> vm_flags == 0 -> no 
code changes needed :)

-- 
Cheers,

David / dhildenb