Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

David Hildenbrand
2022-07-20T19:33:55Z

On 20.07.22 21:15, Peter Xu wrote:
> On Wed, Jul 20, 2022 at 05:10:37PM +0200, David Hildenbrand wrote:
>> For pagecache pages it may as well be *plain wrong* to bypass the write
>> fault handler and simply mark pages dirty+map them writable.
>
> Could you elaborate?

Write-fault handling for some filesystems (that even require this
"slow path") is a bit special.

For example, do_shared_fault() might have to call page_mkwrite().

AFAIK file systems use that for lazy allocation of disk blocks.
If you simply go ahead and map a !dirty pagecache page writable
and mark it dirty, it will not trigger page_mkwrite() and you might
end up corrupting data.

That's why we the old change_pte_range() code never touched
anything if the pte wasn't already dirty. Because as long as it's not writable,
the FS might have to be informed about the write-unprotect.

And we end up in the case here for VM_SHARED with vma_wants_writenotify().
Where we, for example, check

/* The backer wishes to know when pages are first written to? *
if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))$
return 1;

Long story short, we should be really careful with write-fault handler bypasses,
especially when deciding to set dirty bits. For pagecache pages, we have to be
especially careful.

For exclusive anon pages it's mostly ok, because wp_page_reuse()
doesn't really contain that much magic. The only thing to consider for ordinary
mprotect() is that there is -- IMHO -- absolutely no guarantee that someone will
write to a specific write-unprotected page soon. For uffd-wp-unprotect it might be
easier to guess, especially, if we un-protect only a single page.

--
Thanks,

David / dhildenb