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

[Nadav Amit <nadav.amit@gmail.com>]

On Jul 20, 2022, at 12:55 PM, David Hildenbrand <david@redhat.com> wrote:

> On 20.07.22 21:48, Peter Xu wrote:
>> On Wed, Jul 20, 2022 at 09:33:35PM +0200, David Hildenbrand wrote:
>>> 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.
>> 
>> I don't think that pte_dirty() check was for the pagecache code. For any fs
>> that has page_mkwrite() defined, it'll already have vma_wants_writenotify()
>> return 1, so we'll never try to add write bit, hence we'll never even try
>> to check pte_dirty().
> 
> I might be too tired, but the whole reason we had this magic before my
> commit in place was only for the pagecache.
> 
> With vma_wants_writenotify()=0 you can directly map the pages writable
> and don't have to do these advanced checks here. In a writable
> MAP_SHARED VMA you'll already have pte_write().
> 
> We only get !pte_write() in case we have vma_wants_writenotify()=1 ...
> 
>  try_change_writable = vma_wants_writenotify(vma, vma->vm_page_prot);
> 
> and that's the code that checked the dirty bit after all to decide --
> amongst other things -- if we can simply map it writable without going
> via the write fault handler and triggering do_shared_fault() .
> 
> See crazy/ugly FOLL_FORCE code in GUP that similarly checks the dirty bit.

I thought you want to get rid of it at least for anonymous pages. No?

> 
> But yeah, it's all confusing so I might just be wrong regarding
> pagecache pages.

Just to note: I am not very courageous and I did not intend to change
condition for when non-anonymous pages are set as writable. That’s the
reason I did not change the dirty for non-writable non-anonymous entries (as
Peter said). And that’s the reason that setting the dirty bit (at least as I
should have done it) is only performed after we made the decision on the
write-bit.

IOW, after you made your decision about the write-bit, then and only then
you may be able to set the dirty bit for writable entries. Since the entry
is already writeable (i.e., can be written without a fault later directly
from userspace), there should be no concern of correctness when you set it.