Re: Question regarding map count for compound pages

[John Donnelly <john.p.donnelly@oracle.com>]

On 11/15/21 20:47, Harish Mara wrote:
> Background:
> On older kernel?s, we could have our device driver create char devices and
> implement file_operations and vm_operations for open, release, mmap, fault
> etc. The driver allocates memory (128KB, order 5) as compound pages.
> The user application would then map/mmap these device files to perform
> read/write operations.
> 
> On recent kernels, this creates problems when the user space maps multiple
> 128KB chunks that exceed 2MB. This would sometime result in bad page
> getting mapped to the user space process. Almost always we see ?Bad page
> map? errors during munmap because the map count is going below 0.
> 
> It looks like the culprit is zap_pte_range(), which calls
> page_remove_rmap() with the compound_flag = false. As a result, instead of
> decrementing the compound_mapcount for the page the page->_mapcount is
> decremented causing a lot of bad page errors.
> 
> Questions:
> Is this the right usage of compound pages? I.e can I allocate compound
> pages in my kernel driver to be mapped to char device file by a user
> process?
> If yes, then why does it fail on latest kernel when the mmap-ed size
> exceeds 2MB?
> If no, why was it working on the older kernels? If it worked then,
> shouldn?t it work now?
> What is special about map size being greater than 2MB to trigger this?
> Should compound pages be used for Anonymous purposes only?
> 
> 
Hi.


It would certainly help if you could isolate when this behavior started 
. Which version x.y.z version works ? And which one fail ?