Re: Avoiding allocation of unused shmem page

David Hildenbrand
2022-10-21T07:23:12Z

On 20.10.22 23:10, Peter Xu wrote:
> On Thu, Oct 20, 2022 at 09:14:09PM +0100, Matthew Wilcox wrote:
>> In yesterday's call, David brought up the case where we fallocate a file
>> in shmem, call mmap(MAP_PRIVATE) and then store to a page which is over
>> a hole. That currently causes shmem to allocate a page, zero-fill it,
>> then COW it, resulting in two pages being allocated when only the
>> COW page really needs to be allocated.
>>
>> The path we currently take through the MM when we take the page fault
>> looks like this (correct me if I'm wrong ...):
>>
>> handle_mm_fault()
>> __handle_mm_fault()
>> handle_pte_fault()
>> do_fault()
>> do_cow_fault()
>> __do_fault()
>> vm_ops->fault()
>>
>> ... which is where we come into shmem_fault(). Apart from the
>> horrendous hole-punch handling case, shmem_fault() is quite simple:
>>
>> err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, SGP_CACHE,
>> gfp, vma, vmf, &ret);
>> if (err)
>> return vmf_error(err);
>> vmf->page = folio_file_page(folio, vmf->pgoff);
>> return ret;
>>
>> What we could do here is detect this case. Something like:
>>
>> enum sgp_type sgp = SGP_CACHE;
>>
>> if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
>> sgp = SGP_READ;
>
> Yes this will start to save the space, but just to mention this may start
> to break anything that will still depend on the pagecache to work. E.g.,
> it'll change behavior if the vma is registered with uffd missing mode;
> we'll start to lose MISSING events for these private mappings. Not sure
> whether there're other side effects.

I don't follow, can you elaborate?

hugetlb doesn't perform this kind of unnecessary allocation and should be fine in regards to uffd. Why should it matter here and how exactly would a problematic sequence look like?

>
> The zero-page approach will not have such issue as long as the pagecache is
> still filled with something.

Having the shared zeropage available would be nice. But I understand that it adds quite some complexity -- on write fault, we have to invalidate all these zeropage mappings.

>
>> err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, sgp, gfp,
>> vma, vmf, &ret);
>> if (err)
>> return vmf_error(err);
>> if (folio)
>> vmf->page = folio_file_page(folio, vmf->pgoff);
>> else
>> vmf->page = NULL;
>> return ret;
>>
>> and change do_cow_fault() like this:
>>
>> +++ b/mm/memory.c
>> @@ -4575,12 +4575,17 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>> if (ret & VM_FAULT_DONE_COW)
>> return ret;
>>
>> - copy_user_highpage(vmf->cow_page, vmf->page, vmf->address, vma);
>> + if (vmf->page)
>> + copy_user_highpage(vmf->cow_page, vmf->page, vmf->address, vma);
>> + else
>> + clear_user_highpage(vmf->cow_page, vmf->address);
>> __SetPageUptodate(vmf->cow_page);
>>
>> ret |= finish_fault(vmf);
>> - unlock_page(vmf->page);
>> - put_page(vmf->page);
>> + if (vmf->page) {
>> + unlock_page(vmf->page);
>> + put_page(vmf->page);
>> + }
>> if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>> goto uncharge_out;
>> return ret;
>>
>> ... I wrote the code directly in my email client; definitely not
>> compile-tested. But if this situation is causing a real problem for
>> someone, this would be a quick fix for them.
>>
>> Is this a real problem or just intellectual curiosity?
>
> For me it's pure curiosity when I was asking this question; I don't have a
> production environment that can directly benefit from this.
>
> For real users I'd expect private shmem will always be mapped on meaningful
> (aka, non-zero) shared pages just to have their own copy, but no better
> knowledge than that.

There is an easy way to trigger this from QEMU, and we've had
customers running into this:

$ grep -E "(Anon|Shmem)" /proc/meminfo
AnonPages: 4097900 kB
Shmem: 1242364 kB

$ qemu-system-x86_64 -object memory-backend-memfd,id=tmp,share=off,size=4G,prealloc=on -S --nographic &

$ grep -E "(Anon|Shmem)" /proc/meminfo
AnonPages: 8296696 kB
Shmem: 5434800 kB

I recall it's fairly easy to get wrong from Libvirt when starting a VM.

We use an empty memfd and map it private. Each page we touch (especially write to)
ends up allocating shmem memory.

Note that figuring out the write side ("write to hole via private mapping") is only
part of the story. For example, by dumping/migrating the VM (reading all memory) we can easily read
yet unpopulated memory and allocate a shmem page as well; once the VM would write to it, we'd
allocate an additional private page.

We'd need support for the shared zeropage to handle that better -- which would implicitly also handle
shared mappings of shmem better -- dumping/migrating a VM would not allocate a lot of shmem pages filled
with zeroes.

--
Thanks,

David / dhildenb