Re: [PATCH V2 0/6] VA to numa node information

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On 9/13/2018 5:25 PM, Dave Hansen wrote:
On 09/13/2018 05:10 PM, Andrew Morton wrote:
Also, VMAs having THP pages can have a mix of 4k pages and hugepages.
The page walks would be efficient in scanning and determining if it is
a THP huge page and step over it. Whereas using the API, the application
would not know what page size mapping is used for a given VA and so would
have to again scan the VMA in units of 4k page size.

If this sounds reasonable, I can add it to the commit / patch description.
As we are judging whether this is a "good" interface, can you tell us a
bit about its scalability?  For instance, let's say someone has a 1TB
VMA that's populated with interleaved 4k pages.  How much data comes
out?  How long does it take to parse?  Will we effectively deadlock the
system if someone accidentally cat's the wrong /proc file?

For the worst case scenario you describe, it would be one line(range) for each 4k. Which would be similar to what you get with '/proc/*/pagemap'. The amount of data copied out at a time is based on the buffer size used in the kernel. Which is 1024. That is if one line(one range) printed is about 40 bytes(char), that means about 25 lines per copy out. Main concern would be holding 'mmap_sem' lock, which can cause hangs. When the 1024 buffer gets filled the mmap_sem is dropped and the buffer content is copied out to the user buffer. Then the mmap_sem lock is reacquired and the page walk continues as needed until the specified user
buffer size is filed or till end of process address space is reached.

One potential issue could be that there is a large VA range with all pages populated from one numa node, then the page walk could take longer while holding mmap_sem lock. This can be addressed by dropping and re-acquiring the mmap_sem lock after certain number of pages have been walked(Say 512 - which is what happens in '/proc/*/pagemap' case).


/proc seems like a really simple way to implement this, but it seems a
*really* odd choice for something that needs to collect a large amount
of data.  The lseek() stuff is a nice addition, but I wonder if it's
unwieldy to use in practice.  For instance, if you want to read data for
the VMA at 0x1000000 you lseek(fd, 0x1000000, SEEK_SET, right?  You read
~20 bytes of data and then the fd is at 0x1000020.  But, you're getting
data out at the next read() for (at least) the next page, which is also
available at 0x1001000.  Seems funky.  Do other /proc files behave this way?

Yes, SEEK_SET to the VA. The lseek offset is the process VA. So it is not going to be different from reading a normal text file. Expect that /proc files are special. Ex In /proc/*/pagemap' file case, read enforces that seek/file offset and the user buffer size passed in to be a multiple of the pagemap_entry_t size or else the read would fail.

The usage for numa_vamaps file will be to SEEK_SET to the VA from where VA range
to numa node information needs to be read.

The 'fd' offset is not taken into consideration here, just the VA. Say each va range to numa node id printed is about 40 bytes(chars). Now if the read only read 20 bytes, it would have read part of the line. Subsequent read would read the remaining bytes of the line, which will
be stored in the kernel buffer.





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