Re: madvise(MADV_COLLAPSE) fails with EINVAL on dirty file-backed text pages

["Garg, Shivank" <shivankg@amd.com>]

On 11/7/2025 3:40 PM, Lorenzo Stoakes wrote:
> On Fri, Nov 07, 2025 at 10:12:02AM +0100, David Hildenbrand (Red Hat) wrote:
>>
>>>
>>> 5. Yes, I'm calling madvise(MADV_COLLAPSE) on the text portion of the executable, using the address
>>>     range obtained from /proc/self/maps. IIUC, this should benefit applications by reducing ITLB pressure.
>>>
>>> I agree with the suggestions to either Return EAGAIN instead of EINVAL or At minimum, document the
>>> EINVAL return for dirty pages. I'm happy to work on a patch.
>>
>> Of course, we could detect that we are in MADV_COLLAPSE and simply writeback ourselves. After all,
>> user space asked for a collapse, and it's not khugepaged that will simple revisit it later.
>>
>> I did something similar in
>>
>> commit ab73b29efd36f8916c6cc9954e912c4723c9a1b0
>> Author: David Hildenbrand <david@redhat.com>
>> Date:   Fri May 16 14:39:46 2025 +0200
>>
>>     s390/uv: Improve splitting of large folios that cannot be split while dirty
>>     Currently, starting a PV VM on an iomap-based filesystem with large
>>     folio support, such as XFS, will not work. We'll be stuck in
>>     unpack_one()->gmap_make_secure(), because we can't seem to make progress
>>     splitting the large folio.
>>
>> Where I effectively use filemap_write_and_wait_range().
>>
>> It could be used early to writeback the whole range to collapse once, possibly.
> 
> I agree, let's just do a sync flush unconditionally and fix this that way.
> 
> This is simpler than I thought, the key bit of information is that we have
> freshly written the executable so it sits in the page cache but dirty.
> 
> Thanks, Lorenzo


Thanks David for sharing the commit. This worked for me and fix is simple.

+        if (!is_shmem && !cc->is_khugepaged && mapping_can_writeback(mapping)) {
+                loff_t range_start = start << PAGE_SHIFT;
+                loff_t range_end = (end << PAGE_SHIFT) - 1;
+                int ret;
+
+                ret = filemap_write_and_wait_range(mapping, range_start, range_end);
+                if (ret) {
+                        result = SCAN_FAIL;
+                        goto out;
+                }
+        }

I'll do some more testing and post a cleaned-up version with proper comments; rebase on mm-next.
Thanks,
Shivank