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

[Zi Yan]

On 13 Apr 2026, at 16:20, Matthew Wilcox wrote:

> 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.

The goal is to catch these issues during development. VM_BUG_ON crashes
the system and that is too much for such issues in collapse_file().

>
>> +	/*
>> +	 * 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?

I think madvise means MADV_HUGEPAGE for the region, not MADV_COLLAPSE.

In v1, I did the check for shmem, but that regressed MADV_COLLAPSE, which
always can collapse THPs on shmem. I know it sounds unreasonable, but
that ship has sailed.

Best Regards,
Yan, Zi