Re: [PATCH] f2fs: do not support mmap write for large folio

Re: [PATCH] f2fs: do not support mmap write for large folio

[Christoph Hellwig]

On Mon, Apr 06, 2026 at 03:49:40PM +0000, Jaegeuk Kim wrote:
> Let's check mmmap writes onto the large folio.

Why?  And how is this not breaking applications?

> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> ---
>  fs/f2fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 2c4880f24b54..edfc3a374c40 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -82,7 +82,7 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
>  	int err = 0;
>  	vm_fault_t ret;
>  
> -	if (unlikely(IS_IMMUTABLE(inode)))
> +	if (mapping_large_folio_support(inode->i_mapping))
>  		return VM_FAULT_SIGBUS;
>  
>  	if (is_inode_flag_set(inode, FI_COMPRESS_RELEASED)) {
> -- 
> 2.53.0.1213.gd9a14994de-goog
> 
> 
---end quoted text---

Re: [PATCH] f2fs: do not support mmap write for large folio

[Jaegeuk Kim]

On 04/06, Christoph Hellwig wrote:
> On Mon, Apr 06, 2026 at 03:49:40PM +0000, Jaegeuk Kim wrote:
> > Let's check mmmap writes onto the large folio.
> 
> Why?  And how is this not breaking applications?

Since we only support the large folio on the read case.

> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > ---
> >  fs/f2fs/file.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 2c4880f24b54..edfc3a374c40 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -82,7 +82,7 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
> >  	int err = 0;
> >  	vm_fault_t ret;
> >  
> > -	if (unlikely(IS_IMMUTABLE(inode)))
> > +	if (mapping_large_folio_support(inode->i_mapping))
> >  		return VM_FAULT_SIGBUS;
> >  
> >  	if (is_inode_flag_set(inode, FI_COMPRESS_RELEASED)) {
> > -- 
> > 2.53.0.1213.gd9a14994de-goog
> > 
> > 
> ---end quoted text---

Re: [PATCH] f2fs: do not support mmap write for large folio

[Christoph Hellwig]

On Tue, Apr 07, 2026 at 10:58:11PM +0000, Jaegeuk Kim wrote:
> On 04/06, Christoph Hellwig wrote:
> > On Mon, Apr 06, 2026 at 03:49:40PM +0000, Jaegeuk Kim wrote:
> > > Let's check mmmap writes onto the large folio.
> > 
> > Why?  And how is this not breaking applications?
> 
> Since we only support the large folio on the read case.

In general spelling such basic out in the commit log, and even comments
is really helpful.  I'm curious how this works, though - by the time
you read a large folio you don't know if it will ever be written to.

Re: [PATCH] f2fs: do not support mmap write for large folio

[David Hildenbrand (Arm)]

On 4/8/26 07:03, Christoph Hellwig wrote:
> On Tue, Apr 07, 2026 at 10:58:11PM +0000, Jaegeuk Kim wrote:
>> On 04/06, Christoph Hellwig wrote:
>>>
>>> Why?  And how is this not breaking applications?
>>
>> Since we only support the large folio on the read case.
> 
> In general spelling such basic out in the commit log, and even comments
> is really helpful.  I'm curious how this works, though - by the time
> you read a large folio you don't know if it will ever be written to.

Why are only large folios supported for read?

Where is that allocation logic and how can that path even be triggered?

Also, usually we check for large folios by testing the actual folio, not
whether the mapping supports them?

-- 
Cheers,

David

Re: [PATCH] f2fs: do not support mmap write for large folio

[Jaegeuk Kim]

On 04/08, David Hildenbrand (Arm) wrote:
> On 4/8/26 07:03, Christoph Hellwig wrote:
> > On Tue, Apr 07, 2026 at 10:58:11PM +0000, Jaegeuk Kim wrote:
> >> On 04/06, Christoph Hellwig wrote:
> >>>
> >>> Why?  And how is this not breaking applications?
> >>
> >> Since we only support the large folio on the read case.
> > 
> > In general spelling such basic out in the commit log, and even comments
> > is really helpful.  I'm curious how this works, though - by the time
> > you read a large folio you don't know if it will ever be written to.
> 
> Why are only large folios supported for read?
> 
> Where is that allocation logic and how can that path even be triggered?
> 
> Also, usually we check for large folios by testing the actual folio, not
> whether the mapping supports them?

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v7.0-rc7&id=05e65c14ea59a401cec4284e9d612f9d5dc1b3f8

Currently I think it's simple to check the mapping in our case.

> 
> -- 
> Cheers,
> 
> David

Re: [PATCH v3] f2fs: do not support mmap write for large folio

[Christoph Hellwig]

[adding back fsdevel and mm to the discussion]

On Wed, Apr 08, 2026 at 06:13:06PM +0000, Jaegeuk Kim wrote:
> Let's check mmap writes onto the large folio, since we don't support writing
> large folios.

What does this fix?  As state in the previous round of discussion f2fs
only support large folios for immutable files, how do you get ->mkwrite
called for them.

The only way I can think of that is when clearing the immutable flag,
but simply rejecting writes with SIGBUS after that seems like it's
breaking the expected API, and I don't see handling for normal writes
after that either.