Re: [f2fs-dev] [PATCH v2] f2fs: another way to set large folio by remembering inode number

[Randy Dunlap]

On 5/25/26 6:10 PM, Jaegeuk Kim wrote:
> On 05/22, Theodore Tso wrote:
>> On Fri, May 22, 2026 at 05:08:41PM +0000, Jaegeuk Kim wrote:
>>>
>>> Thank you for the explanation. It seems I made a wrong assumption on the
>>> usage of "user." prefix where each filesystem can support in different
>>> ways.
>>
>> The "user." prefix is used by all userspace applications that wish to
>> store extended attributes.  For example, user.mime_type,
>> user.xdg.origin_url, user.charset, user.appache_handler, etc
>>
>> For more information, see:
>>
>>     https://www.freedesktop.org/wiki/CommonExtendedAttribute
>>     https://wiki.archlinux.org/title/Extended_attributes
>>
>> I certainly assumed this was common knowledge across all file system
>> maintainers, but this was apparently not true in your case.  I don't
>> know how this could be the case given that f2fs implements extended
>> attributes, and I would have thought you would have known that when
>> testing that feature.
>>
>>> I shared some motivation when replying to Darrick's feedback [1], but yes,
>>> it was not enough for all heads-up. The problem started that some speicific
>>> application needs as many high-order pages as possible mostly for reads. So,
>>> I thought we can turn on large folio on the specific files per hints. One way
>>> for the hints was using immutable bit, but it turned out it's very hard to
>>> manage disabling the bit whenever deleting the files. Along with limited
>>> ioctl() and requiring inode eviction to manage large folio activation, I had
>>> to implement this path.
>>>
>>> [1] https://lore.kernel.org/lkml/aeA5C8byIpXWla7f@xxxxxxxxxx/
>>
>> Actually, you still haven't explained your use case, at least, not
>> well enough for me to understand what you are trying to do.
>>
>> So an application wants a particular file to use as many high-order
>> pages as possible.  Why?  What sort of guarantees do you need to
>> provide?  What happens if they can't be provided?  What happens if a
>> possibly malicious, or at least gready, application uses this
>> interface to grab a lot of high-order pages?
>>
>> >From your patch:
>>
>> 1. setxattr(file, "user.fadvise", &value, sizeof(unsigned int), 0)
>>  -> register the inode number for large folio
>> 2. chmod(0400, file)
>>  -> make Read-Only
>> 3. open()
>>  -> f2fs_iget() with large folio
>> 4. open(WRITE), mkwrite on mmap, chmod(WRITE)
>>  -> return error
>> 5. iput() and open()
>>  -> goto #3
>> 6. unlink
>>  -> deregister the inode number
>>
>> Why should making the file read-only matter?  And when you say
>> "derigster the inode number", why should this be related to deleting
>> the inode?
>>
>> This is an interface which seems to be very specific to your use case.
>> What if those requirements change over time?  What if you want pull in
>> a file without making it be read-only?  And what if you want to
>> release the large-order pages without deleting the file?
> 
> Let me try to write more details, helped with Gemini.

[as an interested reader:]

If this idea is so good, why shouldn't it be done in the VFS/MM so that
other filesystems could do the same thing instead of just in f2fs?


-- 
~Randy