[PATCH v3] mm: shmem: always support large folios for internal shmem mount

[PATCH v3] mm: shmem: always support large folios for internal shmem mount

[Baolin Wang]

Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
determine whether the shmem mount supports large folios. However, for anonymous
shmem, whether it supports large folios can be dynamically configured via sysfs
interfaces, so setting or not setting mapping_set_large_folios() during initialization
cannot accurately reflect whether anonymous shmem actually supports large folios,
which has already caused some confusion[1].

As discussed with David[2], for anonymous shmem we can treat it as always potentially
having large folios. Therefore, always support large folios for the internal shmem
mount (e.g., anonymous shmem), and which large order allocations are allowed can be
configured dynamically via the 'shmem_enabled' interfaces.

[1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
[2] https://lore.kernel.org/all/875dc63b-0cd2-49e5-8b0d-3fb062789813@kernel.org/
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from v2:
 - Always support large folios for internal shmem mount, per David.
Changes from v1:
 - Update the comments and commit message, per Lance.
---
 mm/shmem.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ecefe02881d..769ef37b1ea9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
 	if (sbinfo->noswap)
 		mapping_set_unevictable(inode->i_mapping);
 
-	/* Don't consider 'deny' for emergencies and 'force' for testing */
-	if (sbinfo->huge)
+	/*
+	 * Always support large folios for the internal shmem mount (e.g.,
+	 * anonymous shmem), and which large order allocations are allowed
+	 * can be configured dynamically via the 'shmem_enabled' interfaces.
+	 *
+	 * For tmpfs, honour the 'huge=' mount option to determine whether
+	 * large folios are supported.
+	 *
+	 * Note: don't consider 'deny' for emergencies and 'force' for testing.
+	 */
+	if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
 		mapping_set_large_folios(inode->i_mapping);
 
 	switch (mode & S_IFMT) {
-- 
2.47.3

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[David Hildenbrand (Arm)]

On 4/17/26 05:25, Baolin Wang wrote:
> Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
> determine whether the shmem mount supports large folios. However, for anonymous
> shmem, whether it supports large folios can be dynamically configured via sysfs
> interfaces, so setting or not setting mapping_set_large_folios() during initialization
> cannot accurately reflect whether anonymous shmem actually supports large folios,
> which has already caused some confusion[1].
> 
> As discussed with David[2], for anonymous shmem we can treat it as always potentially
> having large folios. Therefore, always support large folios for the internal shmem
> mount (e.g., anonymous shmem), and which large order allocations are allowed can be
> configured dynamically via the 'shmem_enabled' interfaces.
> 
> [1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
> [2] https://lore.kernel.org/all/875dc63b-0cd2-49e5-8b0d-3fb062789813@kernel.org/
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Changes from v2:
>  - Always support large folios for internal shmem mount, per David.
> Changes from v1:
>  - Update the comments and commit message, per Lance.
> ---
>  mm/shmem.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4ecefe02881d..769ef37b1ea9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>  	if (sbinfo->noswap)
>  		mapping_set_unevictable(inode->i_mapping);
>  
> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
> -	if (sbinfo->huge)
> +	/*
> +	 * Always support large folios for the internal shmem mount (e.g.,
> +	 * anonymous shmem), and which large order allocations are allowed
> +	 * can be configured dynamically via the 'shmem_enabled' interfaces.
> +	 *
> +	 * For tmpfs, honour the 'huge=' mount option to determine whether
> +	 * large folios are supported.
> +	 *
> +	 * Note: don't consider 'deny' for emergencies and 'force' for testing.
> +	 */
> +	if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>  		mapping_set_large_folios(inode->i_mapping);

Two questions from a non-fs person about the semantics here:

a) Can sbinfo->huge be triggered later, for example, through a remount
(staring at shmem_reconfigure())

b) Do we cover all cases with the SB_KERNMOUNT where sbinfo->huge cannot
be changed later?

-- 
Cheers,

David

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[Baolin Wang]

On 4/17/26 5:21 PM, David Hildenbrand (Arm) wrote:
> On 4/17/26 05:25, Baolin Wang wrote:
>> Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
>> determine whether the shmem mount supports large folios. However, for anonymous
>> shmem, whether it supports large folios can be dynamically configured via sysfs
>> interfaces, so setting or not setting mapping_set_large_folios() during initialization
>> cannot accurately reflect whether anonymous shmem actually supports large folios,
>> which has already caused some confusion[1].
>>
>> As discussed with David[2], for anonymous shmem we can treat it as always potentially
>> having large folios. Therefore, always support large folios for the internal shmem
>> mount (e.g., anonymous shmem), and which large order allocations are allowed can be
>> configured dynamically via the 'shmem_enabled' interfaces.
>>
>> [1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
>> [2] https://lore.kernel.org/all/875dc63b-0cd2-49e5-8b0d-3fb062789813@kernel.org/
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Changes from v2:
>>   - Always support large folios for internal shmem mount, per David.
>> Changes from v1:
>>   - Update the comments and commit message, per Lance.
>> ---
>>   mm/shmem.c | 13 +++++++++++--
>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 4ecefe02881d..769ef37b1ea9 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>>   	if (sbinfo->noswap)
>>   		mapping_set_unevictable(inode->i_mapping);
>>   
>> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
>> -	if (sbinfo->huge)
>> +	/*
>> +	 * Always support large folios for the internal shmem mount (e.g.,
>> +	 * anonymous shmem), and which large order allocations are allowed
>> +	 * can be configured dynamically via the 'shmem_enabled' interfaces.
>> +	 *
>> +	 * For tmpfs, honour the 'huge=' mount option to determine whether
>> +	 * large folios are supported.
>> +	 *
>> +	 * Note: don't consider 'deny' for emergencies and 'force' for testing.
>> +	 */
>> +	if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>>   		mapping_set_large_folios(inode->i_mapping);
> 
> Two questions from a non-fs person about the semantics here:
> 
> a) Can sbinfo->huge be triggered later, for example, through a remount
> (staring at shmem_reconfigure())

For tmpfs, yes.

> b) Do we cover all cases with the SB_KERNMOUNT where sbinfo->huge cannot
> be changed later?

For mounts with the SB_KERNMOUNT flag set, which is essentially the 
internal shmem mount, as we discussed, we don't care about sbinfo->huge. 
Because for the internal shmem mount, we always consider it as 
potentially having large folios.

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[David Hildenbrand (Arm)]

On 4/17/26 11:27, Baolin Wang wrote:
> 
> 
> On 4/17/26 5:21 PM, David Hildenbrand (Arm) wrote:
>> On 4/17/26 05:25, Baolin Wang wrote:
>>> Currently, when shmem mounts are initialized, they only use 'sbinfo-
>>> >huge' to
>>> determine whether the shmem mount supports large folios. However, for
>>> anonymous
>>> shmem, whether it supports large folios can be dynamically configured
>>> via sysfs
>>> interfaces, so setting or not setting mapping_set_large_folios()
>>> during initialization
>>> cannot accurately reflect whether anonymous shmem actually supports
>>> large folios,
>>> which has already caused some confusion[1].
>>>
>>> As discussed with David[2], for anonymous shmem we can treat it as
>>> always potentially
>>> having large folios. Therefore, always support large folios for the
>>> internal shmem
>>> mount (e.g., anonymous shmem), and which large order allocations are
>>> allowed can be
>>> configured dynamically via the 'shmem_enabled' interfaces.
>>>
>>> [1] https://lore.kernel.org/all/
>>> ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
>>> [2] https://lore.kernel.org/
>>> all/875dc63b-0cd2-49e5-8b0d-3fb062789813@kernel.org/
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>> Changes from v2:
>>>   - Always support large folios for internal shmem mount, per David.
>>> Changes from v1:
>>>   - Update the comments and commit message, per Lance.
>>> ---
>>>   mm/shmem.c | 13 +++++++++++--
>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> index 4ecefe02881d..769ef37b1ea9 100644
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct
>>> mnt_idmap *idmap,
>>>       if (sbinfo->noswap)
>>>           mapping_set_unevictable(inode->i_mapping);
>>>   -    /* Don't consider 'deny' for emergencies and 'force' for
>>> testing */
>>> -    if (sbinfo->huge)
>>> +    /*
>>> +     * Always support large folios for the internal shmem mount (e.g.,
>>> +     * anonymous shmem), and which large order allocations are allowed
>>> +     * can be configured dynamically via the 'shmem_enabled'
>>> interfaces.
>>> +     *
>>> +     * For tmpfs, honour the 'huge=' mount option to determine whether
>>> +     * large folios are supported.
>>> +     *
>>> +     * Note: don't consider 'deny' for emergencies and 'force' for
>>> testing.
>>> +     */
>>> +    if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>>>           mapping_set_large_folios(inode->i_mapping);
>>
>> Two questions from a non-fs person about the semantics here:
>>
>> a) Can sbinfo->huge be triggered later, for example, through a remount
>> (staring at shmem_reconfigure())
> 
> For tmpfs, yes.

So, we could pass this check here, not setting
mapping_set_large_folios(), but later someone toggles it and we missed
to set mapping_set_large_folios()?

Or would we always go through another __shmem_get_inode() after a remount?

-- 
Cheers,

David

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[Baolin Wang]

On 4/17/26 5:52 PM, David Hildenbrand (Arm) wrote:
> On 4/17/26 11:27, Baolin Wang wrote:
>>
>>
>> On 4/17/26 5:21 PM, David Hildenbrand (Arm) wrote:
>>> On 4/17/26 05:25, Baolin Wang wrote:
>>>> Currently, when shmem mounts are initialized, they only use 'sbinfo-
>>>>> huge' to
>>>> determine whether the shmem mount supports large folios. However, for
>>>> anonymous
>>>> shmem, whether it supports large folios can be dynamically configured
>>>> via sysfs
>>>> interfaces, so setting or not setting mapping_set_large_folios()
>>>> during initialization
>>>> cannot accurately reflect whether anonymous shmem actually supports
>>>> large folios,
>>>> which has already caused some confusion[1].
>>>>
>>>> As discussed with David[2], for anonymous shmem we can treat it as
>>>> always potentially
>>>> having large folios. Therefore, always support large folios for the
>>>> internal shmem
>>>> mount (e.g., anonymous shmem), and which large order allocations are
>>>> allowed can be
>>>> configured dynamically via the 'shmem_enabled' interfaces.
>>>>
>>>> [1] https://lore.kernel.org/all/
>>>> ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
>>>> [2] https://lore.kernel.org/
>>>> all/875dc63b-0cd2-49e5-8b0d-3fb062789813@kernel.org/
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> ---
>>>> Changes from v2:
>>>>    - Always support large folios for internal shmem mount, per David.
>>>> Changes from v1:
>>>>    - Update the comments and commit message, per Lance.
>>>> ---
>>>>    mm/shmem.c | 13 +++++++++++--
>>>>    1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index 4ecefe02881d..769ef37b1ea9 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct
>>>> mnt_idmap *idmap,
>>>>        if (sbinfo->noswap)
>>>>            mapping_set_unevictable(inode->i_mapping);
>>>>    -    /* Don't consider 'deny' for emergencies and 'force' for
>>>> testing */
>>>> -    if (sbinfo->huge)
>>>> +    /*
>>>> +     * Always support large folios for the internal shmem mount (e.g.,
>>>> +     * anonymous shmem), and which large order allocations are allowed
>>>> +     * can be configured dynamically via the 'shmem_enabled'
>>>> interfaces.
>>>> +     *
>>>> +     * For tmpfs, honour the 'huge=' mount option to determine whether
>>>> +     * large folios are supported.
>>>> +     *
>>>> +     * Note: don't consider 'deny' for emergencies and 'force' for
>>>> testing.
>>>> +     */
>>>> +    if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>>>>            mapping_set_large_folios(inode->i_mapping);
>>>
>>> Two questions from a non-fs person about the semantics here:
>>>
>>> a) Can sbinfo->huge be triggered later, for example, through a remount
>>> (staring at shmem_reconfigure())
>>
>> For tmpfs, yes.
> 
> So, we could pass this check here, not setting
> mapping_set_large_folios(), but later someone toggles it and we missed
> to set mapping_set_large_folios()?

Indeed. Good point.

> 
> Or would we always go through another __shmem_get_inode() after a remount?

Not really. There could be files created before remount whose mappings 
don't support large folios (with 'huge=never' option), while files 
created after remount will have mappings that support large folios (if 
remounted with 'huge=always' option).

It looks like the previous commit 5a90c155defa was also problematic. The 
huge mount option has introduced a lot of tricky issues:(

Now I think Zi's previous suggestion should be able to clean up this 
mess? That is, calling mapping_set_large_folios() unconditionally for 
all shmem mounts, and revisiting Kefeng's first version to fix the 
performance issue.

[1] 
https://lore.kernel.org/all/20240914140613.2334139-1-wangkefeng.wang@huawei.com/

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[David Hildenbrand (Arm)]

On 4/17/26 14:45, Baolin Wang wrote:
> 
> 
> On 4/17/26 5:52 PM, David Hildenbrand (Arm) wrote:
>> On 4/17/26 11:27, Baolin Wang wrote:
>>>
>>>
>>>
>>> For tmpfs, yes.
>>
>> So, we could pass this check here, not setting
>> mapping_set_large_folios(), but later someone toggles it and we missed
>> to set mapping_set_large_folios()?
> 
> Indeed. Good point.
> 
>>
>> Or would we always go through another __shmem_get_inode() after a
>> remount?
> 
> Not really. There could be files created before remount whose mappings
> don't support large folios (with 'huge=never' option), while files
> created after remount will have mappings that support large folios (if
> remounted with 'huge=always' option).
> 
> It looks like the previous commit 5a90c155defa was also problematic. The
> huge mount option has introduced a lot of tricky issues:(
> 
> Now I think Zi's previous suggestion should be able to clean up this
> mess? That is, calling mapping_set_large_folios() unconditionally for
> all shmem mounts, and revisiting Kefeng's first version to fix the
> performance issue.

Okay, so you'll send a patch to just set mapping_set_large_folios()
unconditionally?

> 
> [1] https://lore.kernel.org/all/20240914140613.2334139-1-
> wangkefeng.wang@huawei.com/

Is that really required? Which call path would be the problematic bit
with the above?

I'd say, we'd check in the large folio allocation code whether ->huge is
set to never instead?

-- 
Cheers,

David

Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

[Baolin Wang]

On 4/21/26 3:00 AM, David Hildenbrand (Arm) wrote:
> On 4/17/26 14:45, Baolin Wang wrote:
>>
>>
>> On 4/17/26 5:52 PM, David Hildenbrand (Arm) wrote:
>>> On 4/17/26 11:27, Baolin Wang wrote:
>>>>
>>>>
>>>>
>>>> For tmpfs, yes.
>>>
>>> So, we could pass this check here, not setting
>>> mapping_set_large_folios(), but later someone toggles it and we missed
>>> to set mapping_set_large_folios()?
>>
>> Indeed. Good point.
>>
>>>
>>> Or would we always go through another __shmem_get_inode() after a
>>> remount?
>>
>> Not really. There could be files created before remount whose mappings
>> don't support large folios (with 'huge=never' option), while files
>> created after remount will have mappings that support large folios (if
>> remounted with 'huge=always' option).
>>
>> It looks like the previous commit 5a90c155defa was also problematic. The
>> huge mount option has introduced a lot of tricky issues:(
>>
>> Now I think Zi's previous suggestion should be able to clean up this
>> mess? That is, calling mapping_set_large_folios() unconditionally for
>> all shmem mounts, and revisiting Kefeng's first version to fix the
>> performance issue.
> 
> Okay, so you'll send a patch to just set mapping_set_large_folios()
> unconditionally?

I'm still hesitating on this. If we set mapping_set_large_folios() 
unconditionally, we need to re-fix the performance regression that was 
addressed by commit 5a90c155defa.

But it's hard for me to convince myself to add a new flag similar to 
IOCB_NO_LARGE_CHUNK for this hack (like the patch in [1] does).

>> [1] https://lore.kernel.org/all/20240914140613.2334139-1-
>> wangkefeng.wang@huawei.com/
> 
> Is that really required? Which call path would be the problematic bit
> with the above?
> 
> I'd say, we'd check in the large folio allocation code whether ->huge is
> set to never instead?

Yes, this is exactly our current logic. When allocating large folios, 
we'll check the ->huge setting in shmem_huge_global_enabled(), which 
means large folio allocations always respect the ->huge setting.

But as I mentioned earlier, the ->huge setting cannot keep the 
mapping_set_large_folios() setting consistent across all mappings in the 
entire tmpfs mount. My concern is that under the same tmpfs mount, after 
remount, we might end up with some mappings supporting large folios 
(calling mapping_set_large_folios()) while others don't.

However, I got some insights from 
Documentation/admin-guide/mm/transhuge.rst. Does this mean that after 
remount, whether the mappings of existing files support large folios 
should remain unchanged?

“
``mount -o remount,huge= /mountpoint`` works fine after mount: 
remounting ``huge=never`` will not attempt to break up huge pages at 
all, just stop more from being allocated.
”

Do you think this makes sense?