Messages in this thread | | | Date | Fri, 09 May 2008 17:33:37 -0500 | From | Timur Tabi <> | Subject | Re: Calling free_pages on part of the memory returned by get_free_pages? |
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Andi Kleen wrote: > Timur Tabi <timur@freescale.com> writes: > >> According to LDD3, if I call get_free_pages() to allocate X bytes, I have to >> free all of those pages with free_pages(). The VM internals are a little bit >> over my head, but I looked at the code and I didn't see why that is a requirement. >> >> For example, let's say I want to allocated 6MB of physically-contiguous memory. >> If I call x = get_free_pages(11) to get 8MB. What happens if I then do >> "free_pages(x + 6 * 1024 * 1024, 9)"? >> >> I remember doing this on the 2.4 kernel, and it never gave me any problems. > > It is ok, as long as you don't use compound pages (__GFP_COMP) and call > split_page() to fix up the reference counts. > > Also you do this to save memory right? The large system hash code does it too > and I used to do it in some 2.4 change with an alloc_pages_exact() > which never made it into 2.6. > > If it's reasonably common we should re-add alloc/get_pages_exact() helper to > make this pattern clear and easier to use.
Follow-up question:
Say I allocate 8MB with __get_free_pages() and then use multiple free_pages() / split_page() calls to free the last 3MB. Will the de-allocated blocks be merged into larger-order chunks, if they're contiguous? That is, if I repeat this process:
1. Allocate 8MB 2. Free 3MB 3. Free 5MB
over and over again, will I totally fragment memory to the point that 8MB allocates will never work again?
-- Timur Tabi Linux kernel developer at Freescale
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