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

["Theodore Tso" <tytso@mit.edu>]

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@google.com/

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?

						- Ted