[HELP] mmap fsdax mode pmem to userland with writethrough

[HELP] mmap fsdax mode pmem to userland with writethrough

[Wang Jianchao]

Hi list

We are trying to mmap the file in xfs-dax to userland with writethrough mode,
hope to avoid bandwidth and latency cause by write-allocate in writeback mode
and we can still use the original userland application code w/o using NTstore.

We add following code in the mmap callback of xfs,

vma->vm_page_prot = pgprot_writethrough(vma->vm_page_prot);

But it seems to not work. When I use pcm-memory to check the bandwidth of PMM,
PMM read BW is still there which is nearly equal with PMM write. Even I use
pgprot_noncached, it still not work.

What should I do to implement the writethrough mapping ?

Thanks a million for any help.

Jianchao
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Re: [HELP] mmap fsdax mode pmem to userland with writethrough

[Wang Jianchao]

It was figured out that the track_pfn_insert changed the pgprot.
I tried two ways to workaround it
(1) change the pte in the fault callback directly
(2) add a helper interface of devm_memremap_pages that has a pgprot.

The writethrough mmap seems to success but the final performance is
really bad, just 20M/s per thread....

On 2020/12/14 6:44 下午, Wang Jianchao wrote:
> Hi list
> 
> We are trying to mmap the file in xfs-dax to userland with writethrough mode,
> hope to avoid bandwidth and latency cause by write-allocate in writeback mode
> and we can still use the original userland application code w/o using NTstore.
> 
> We add following code in the mmap callback of xfs,
> 
> vma->vm_page_prot = pgprot_writethrough(vma->vm_page_prot);
> 
> But it seems to not work. When I use pcm-memory to check the bandwidth of PMM,
> PMM read BW is still there which is nearly equal with PMM write. Even I use
> pgprot_noncached, it still not work.
> 
> What should I do to implement the writethrough mapping ?
> 
> Thanks a million for any help.
> 
> Jianchao
> 
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