[linux-mm-cc] Re: [lc-devel] Chunk list issues

[Asbjørn Sannes]

Nitin Gupta wrote:
> Hi Anderson,
>
>>
>>> On 6/28/06, Anderson Briglia <briglia.anderson at gmail.com> wrote:
>>> > > > I have a doubt related to chunk operations:
>>> > > >
>>> > > > - I guess you will use the virtual swap as the compressed
>>> cache for
>>> > > > anon pages, right? If yes, how will you swapin/swapout chunks?
>>> If a
>>> > > > single chunk points to a portion of a physical page, and the
>>> > > > swapin/swapout operations handle entire pages of data.
>>> > >
>>> > > Yes, vswap is for anon pages. I think you are mistaken
>>> somewhere...I'm not using
>>> > > bio structs or anything to swapin/out pages from vswap.
>>> >
>>> > Ok. Will be possible to transfer pages from vswap to a 'real' swap
>>> area?
>>> >
>>>
>>> Yes, but 'procedure' will be same as for transferring a page from a
>>> swap to any other swap: change it swp_entry_t to have 'type' for other
>>> swap and it will get written to this second swap.
>>
>> What will be the factor that determines when/which page will be
>> migrated from vswap to real swap?
>>
>
> vswap is full. Maybe others but I haven't thought of them yet :P
>

What should be considered here is how do we actually "migrate" a page
like this? Because this isn't done very often normally (how often do you
add/deactive swap areas) it is my understanding that this is an
expensive process, which is why we can't really do it, a proposal is to
instead of having a "vswap" area, that we allocate other physical swap
areas in advance and do the interception of find_get_page & friends as
is done in the git tree now. This way, when we need to kick out pages it
is a very straightforward process, just put it in the pre-allocated swap
slot. Of course this wastes swap space (probably disk) and doesn't work
with no swap area at all, so maybe a mix where you can add vswap in some
other way for no-swap area (where we never kick out pages anyways)
purely as an swap entry id excercise.

Greetings,
Asbjørn Sannes