Re: brd in a memdesc world

[Pankaj Raghav <p.raghav@samsung.com>]

>>> Currently brd uses page->index as a debugging check.  In the memdesc
>>> future, struct page has no members (you could store a small amount of
>>> information in it, but I'm not willing to commit to more than a few bits).
>>>
>>
>> Shouldn't we change brd to use folios? Once we do that, this will not
>> be a problem any more right?
> 
> We certainly could change brd to use folios.  But why would we want to?
> Hannes' work always allocates memory of a fixed size (a fixed multiple
> of PAGE_SIZE).  Folios are a medium-weight data structure (probably
> about 80 bytes once we get to memdescs).  They support a lot of things,
> eg belonging to an inode, having an index, being mappable to userspace,
> being lockable, accountable to memcgs, allowing extra private data,
> knowing their own size, ...
> 

Got it! Probably moving to folios just for the sake of retaining the
debugging checks is not enough.

> None of those things are needed for brd's uses.  All brd needs is to
> be able to allocate, kmap and free chunks of memory.  Unless there are
> plans to do more than this.
> 

I remember he mentioned he wanted to support bigger logical block sizes
in brd, in which case moving to folios might be justified.