Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Improving large folio writeback performance

[David Hildenbrand <david@redhat.com>]

On 17.01.25 15:17, Matthew Wilcox wrote:
> On Fri, Jan 17, 2025 at 12:56:52PM +0100, Jan Kara wrote:
>> On Fri 17-01-25 12:40:15, Vlastimil Babka wrote:
>>> I think this might be tricky in some cases? I.e. with 2 MB and pmd-mapped
>>> folio, it's possible to write-protect only the whole pmd, not individual 32k
>>> chunks in order to catch the first write to a chunk to mark it dirty.
>>
>> Definitely. Once you map a folio through PMD entry, you have no other
>> option than consider whole 2MB dirty. But with PTE mappings or
>> modifications through syscalls you can do more fine-grained dirtiness
>> tracking and there're enough cases like that that it pays off.
> 
> Almost no applications use shared mmap writes to write to files.  The
> error handling story is crap and there's only limited control about when
> writeback actually happens.  Almost every application uses write(), even
> if they have the file mmaped.  This isn't a scenario worth worrying about.

Right, for example while ordinary files can be used for backing VMs, VMs 
permanently dirty a lot of memory, resulting in a constant writeback 
stream and nasty storage wear (so I've been told). There are some use 
cases for it, but we primarily use shmem/memfd/anon instead.

-- 
Cheers,

David / dhildenb