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SubjectRe: Question about memory mapping mechanism
Martin Drab wrote:
> Hi,
> I'm writing a driver for a sampling device that is constantly delivering a
> relatively high amount of data (about 16 MB/s) and I need to deliver the
> data to the user-space ASAP. To prevent data loss I create a queue of
> buffers (consisting of few pages each) which are more or less directly
> filled by the device and then mapped to the user-space via mmap().
> The thing is that I'd like to prevent kernel to swap these pages out,
> because then I may loose some data when they are not available in time
> for the next round.
> My original idea (that used to work in the past) was to allocate the
> buffers using __get_free_pages(), then pin the pages down by setting their
> PG_reserved bit in the page flags before using them. And then set the
> VM_RESERVED flag of the appropriate VMA when mmap() is called for these
> pages that are then mapped using nopage() mechanism.
> But this way no longer seems to work correctly, it kind of works, but I'm
> getting following messages for each mmapped page upon munmap() call:
> --------------------------------------
> [19172.939248] Bad page state in process 'dtrtest'
> [19172.939249] page:ffff81000160a978 flags:0x001a000000000404 mapping:0000000000000000 mapcount:0 count:0
> [19172.939251] Trying to fix it up, but a reboot is needed
> [19172.939253] Backtrace:
> [19172.939256]
> [19172.939257] Call Trace:
> [19172.939273] [<ffffffff802adc37>] bad_page+0x57/0x90
> [19172.939280] [<ffffffff8020b92f>] free_hot_cold_page+0x7f/0x180
> [19172.939287] [<ffffffff80207a90>] unmap_vmas+0x450/0x750
> [19172.939308] [<ffffffff80212867>] unmap_region+0xb7/0x160
> [19172.939318] [<ffffffff80211918>] do_munmap+0x238/0x2f0
> [19172.939325] [<ffffffff802656c5>] __down_write_nested+0x35/0xf0
> [19172.939334] [<ffffffff80215ffd>] sys_munmap+0x4d/0x80
> [19172.939341] [<ffffffff8025f11e>] system_call+0x7e/0x83
> -------------------------------
> Aparently due to the PG_reserved bit set.
> So my question is: What is currently a proper way to do all this cleanly?

Have you looked at the Infiniband stuff? I know the folks working on
the ipath driver eventually got this kind of thing working in a sane way.

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