Apex Legends game periodically (every 30 seconds) calls this function with up to 22k virtual addresses. All but 1 of them is valid. Due to amount of queries addresses, and cost of seek+read, this causes this function to take up to about 50ms. So framerate drops from ~150 FPS to 20FPS for about a second.
As far as I can see, returning 0 entries from this function, still makes Apex Legend work.
But keep code correct, and optimise it by:
1. Opening pagemap file once, and never closing it. This eliminates repeated fopen/fseek/fread/fclose sequences, which helps even in queries with small amount of virtual addresses.
2. Using pread, instead of seek+read.
3. Only performing pagemap read when the address is valid.
Future work might recognize continues pages in the query, and perform a batch read of multiple pagemap entries, instead one page at a time, but for now it is not necassary.
This change get_working_set_ex peek wall clock runtime from 57ms to 0.29ms.
Tested on Linux, but similar change done for the BSD part.
`Signed-off-by: Witold Baryluk witold.baryluk@gmail.com`
-- v16: ntdll: Keep pagemap file open after first use of NtQueryVirtualMemory(MemoryWorkingSetExInformation)