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Message-ID: <d1812c17-0036-407f-9ac3-207705341d5e@redhat.com>
Date: Tue, 12 Nov 2024 17:21:56 +0100
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
hughd@...gle.com
Cc: willy@...radead.org, wangkefeng.wang@...wei.com, 21cnbao@...il.com,
ryan.roberts@....com, ioworker0@...il.com, da.gomez@...sung.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] mm: shmem: add large folio support for tmpfs
On 12.11.24 17:19, David Hildenbrand wrote:
> 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@...sung.com>
>> Signed-off-by: Daniel Gomez <da.gomez@...sung.com>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.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 :)
... and now I wonder about documented "fadvise", because this here is
only concerned with madvise?
--
Cheers,
David / dhildenb
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