Messages in this thread | | | Subject | VM_RESERVED vs vm_normal_page() | From | Benjamin Herrenschmidt <> | Date | Wed, 13 Dec 2006 15:37:57 +1100 |
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Hi folks !
What is the logic regarding VM_RESERVED, and more specifically, why is vm_normal_page() nor returning NULL for these ?
I have struct pages that are a bit special for things like SPE mappings on Cell and I'd like to avoid a lot of the stuff the VM tries to do on them, like rmap accounting, etc... In fact, for almost everything, the semantics I want are vm_normal_page() to return NULL... I have struct pages because for now I need do_no_page() to work, but I must absolutely avoid things like getting swapped out etc...
Thus I looked at the logic in vm_normal_page() and it really sucks... this is pretty much a "heuristic" to differenciate remap_page_range from copy_on_write stuff ... gack.
What is VM_RESERVED for then ? Could I just use that ? I currently only have VM_IO set on my SPE VMAs but I could add it. The current implementation of vm_normal_page() doesn't test for it though, maybe it should ?
I have still a problem though, in the case vm_normal_page() is made to return NULL...
In that case, unmapping of pages will still cause them to be released (tlb_* -> free_pages_and_swap_cache -> release_pages -> put_page_testzero) but if you fork(), copy_one_pte() will not call an additional get_page() when vm_normal_page() returns NULL... thus I fear the page count will become bocus accross forks...
In the long run, I want to stop using struct page's for the SPU registers and local store, once Nick's new stuff gets in, though I might have a go at hacking something together before... but in the meantime, I'd like to figure out what is the _safe_ way of having a VMA containing PTEs mapping to struct pages that are _not_ normal memory and thus aren't to be swapped out, freed, or anything like that.
Cheers, Ben.
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