Messages in this thread | | | Date | Tue, 30 Apr 2002 11:01:08 +0530 | From | Suparna Bhattacharya <> | Subject | Re: [PATCH]Fix: Init page count for all pages during higher order allocs |
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On Mon, Apr 29, 2002 at 11:40:21AM -0600, Eric W. Biederman wrote: > Suparna Bhattacharya <suparna@in.ibm.com> writes: > > > The call to set_page_count(page, 1) in page_alloc.c appears to happen > > only for the first page, for order 1 and higher allocations. > > This leaves the count for the rest of the pages in that block > > uninitialised. > > Actually it should be zero. > > This is deliberate because high order pages should not be referenced by > their partial pages.
That sounds reasonable provided there is a way to identify the main page struct corresponding to an area that's part of a higher order page.
> It might make sense to add a PG_large flag and > then in the immediately following struct page add a pointer to the next > page, so you can identify these pages by inspection. Doing something > similar to the PG_skip flag.
Maybe different solutions could emerge for this in 2.4 and 2.5.
Even a PG_partial flag for the partial pages will enable us to traverse back to the main page, and vice-versa to determine the partial pages covered by the main page, without any additional pointers. Is that an acceptable option for 2.4 ? (That's one more page flag ...)
It would be good to have a way to determine the order directly from the page struct, without such traversals, at least in 2.5.
> > Beyond that I get nervous, that people will treat it as endorsement of > doing a high order continuous allocation and then fragmenting the page.
I don't think it would amount to such an endorsement. It's just a matter of replicating the settings from the main page to the partial pages - which might be considered an alternate protocol, though a little inefficient for really high orders. However, having the partial page counts zeroed out probably helps as a safeguard in some situations in view of the page count sanity checks. Or are there any scenarios where you forsee a problem/breakage ?
Regards Suparna
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