Re: [PATCH RESEND V2 0/5] Fixes and cleanups to xarray

[Kemeng Shi <shikemeng@huaweicloud.com>]

on 11/12/2024 11:48 PM, Matthew Wilcox wrote:
> On Tue, Nov 12, 2024 at 03:05:51PM +0800, Kemeng Shi wrote:
>> Patch 1 fixes the issue that xas_find_marked() will return a sibling entry
>> to users when there is a race to replace old entry with small range with
>> a new entry with bigger range. The kernel will crash if users use returned
>> sibling entry as a valid entry.
>> For example, find_get_entry() will crash if it gets a sibling entry from
>> xas_find_marked() when trying to search a writeback folios.
> 
> You _think_ this can happen, or you've observed a crash where it _has_
> happened?  I don't think it can happen due to the various locks we
> have.  I'm not opposed to including this patch, but I don't think it
> can happen today.Sorry for confusion. As I mentioned at last email, we need to confirm if
the trigger condition exists in real world of all three potential bug. Here,
I want to describe the potential impact if the potential bug happens for
some early input.
> 
>> Patch 3 fixes the issue that xas_pause() may skip some entry unepxectedly
>> after xas_load()(may be called in xas_for_each for first entry) loads a
>> large entry with index point at mid of the entry. So we may miss to writeback
>> some dirty page and lost user data.
> 
> How do we lose user data?  The inode will still contain dirty pages.
> We might skip some pages we should not have skipped, but they'll be
> caught by a subsequent pass, no?
> 
After flush, users may expect dirty pages are in storage and are persistent. But
if we miss to writeback some dirty pages and kernel is reboot after flush, then
the data in un-written pages are last to users. I'm not sure if I miss something
are. But again, we need to confirm if the trigger condition exists in real world.

Thanks.