Re: Fixing excessive shootdowns during FFS read (kern/53124) on x86 - emap, direct map

[Jaromír Doleček <jaromir.dolecek%gmail.com@localhost>]

2018-05-12 21:24 GMT+02:00 Chuck Silvers <chuq%chuq.com@localhost>:
> the problem with keeping the pages locked (ie. PG_BUSY) while accessing
> the user address space is that it can lead to deadlock if the page

Meanwhile I've tested this scenario - wrote a test program to do
mmap(2) for file, then calling write() using the memory from mmap as
the buffer for write(2) for the same file offset.

It actually caused the process to get SIGSEGV with new code, where the
old code made write(2) just returned with EINVAL. I think this happens
because fault resolution returns EINVAL when the page has PG_BUSY.
Funny thing is that actually on Mac OS X that code actually does end
up with deadlock, and unkillable process :D

Maybe the SIGSEGV is actually acceptable behaviour? I don't think
there would be valid reason to do mmap(2)/write(2) combination like
that.

Jaromir