Re: [PATCH 7.2 v2 01/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Matthew Wilcox]

On Mon, Apr 13, 2026 at 03:20:19PM -0400, Zi Yan wrote:
> collapse_file() requires FSes supporting large folio with at least
> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that.
> MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.
> 
> While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.

Why?  These are bugs.  I don't think we gain anything from continuing.

> +	/*
> +	 * skip files without PMD-order folio support
> +	 * do not check shmem, since MADV_COLLAPSE ignores shmem huge config
> +	 */
> +	if (!shmem_file(file) && mapping_max_folio_order(mapping) < PMD_ORDER)
> +		return SCAN_FAIL;

I wonder if it should.  If the commit message to 5a90c155defa is
to be believed,

    Since 'deny' is for emergencies and 'force' is for testing, performance
    issues should not be a problem in real production environments, so don't
    call mapping_set_large_folios() in __shmem_get_inode() when large folio is
    disabled with mount huge=never option (default policy).

so maybe MADV_COLLAPSE should honour huge=never?
Documentation/filesystems/tmpfs.rst implies that we do!

huge=never       Do not allocate huge pages.  This is the default.
huge=always      Attempt to allocate huge page every time a new page is needed.
huge=within_size Only allocate huge page if it will be fully within i_size.
                 Also respect madvise(2) hints.
huge=advise      Only allocate huge page if requested with madvise(2).

so what's the difference between huge=never and huge=madvise?