Re: [Question] CoW on VM_PFNMAP vma during write fault

[David Hildenbrand]

On 04.03.24 09:47, mawupeng wrote:
> Hi Maintainers, kindly ping...
>
> On 2024/2/28 9:55, mawupeng wrote:
> > On 2024/2/27 21:15, David Hildenbrand wrote:
> > > On 27.02.24 14:00, David Hildenbrand wrote:
> > > > On 27.02.24 13:28, Wupeng Ma wrote:
> > > > > We find that a warn will be produced during our test, the detail log is
> > > > > shown in the end.
> > > > >
> > > > > The core problem of this warn is that the first pfn of this pfnmap vma is
> > > > > cleared during memory-failure. Digging into the source we find that this
> > > > > problem can be triggered as following:
> > > > >
> > > > > // mmap with MAP_PRIVATE and specific fd which hook mmap
> > > > > mmap(MAP_PRIVATE, fd)
> > > > >      __mmap_region
> > > > >        remap_pfn_range
> > > > >        // set vma with pfnmap and the prot of pte is read only
> > > > >      
> > > >
> > > > Okay, so we get a MAP_PRIVATE VM_PFNMAP I assume.
> > > >
> > > > What fd is that exactly? Often, we disallow private mappings in the
> > > > mmap() callback (for a good reason).
>
> We found this problem in 5.10, Commit 9f78bf330a66 ("xsk: support use vaddr as ring") Fix this
> problem during supporting vaddr by remap VM_PFNMAP by VM_MIXEDMAP. But other modules which
> use remap_pfn_range may still have this problem.

I wrote a simple reproducer using MAP_PRIVATE of iouring queues on Friday.

> It do seems wired for private mappings, What is the good reason?

I'm sure there are some use cases that require MAP_PRIVATE of such 
areas, and usually there is nothing wrong with that.

It's just that the PAT implementation incompatible.

I can submit a cleaned-up version of my patches.

--
Cheers,

David / dhildenb