On Tue 12-02-19 16:55:21, Christopher Lameter wrote: > On Tue, 12 Feb 2019, Jan Kara wrote: > > > > Isn't that already racy? If the mmap user is fast enough can't it > > > prevent the page from becoming freed in the first place today? > > > > No, it cannot. We block page faulting for the file (via a lock), tear down > > page tables, free pages and blocks. Then we resume faults and return > > SIGBUS (if the page ends up being after the new end of file in case of > > truncate) or do new page fault and fresh block allocation (which can end > > with SIGBUS if the filesystem cannot allocate new block to back the page). > > Well that is already pretty inconsistent behavior. Under what conditions > is the SIGBUS occurring without the new fault attempt? I probably didn't express myself clearly enough. I didn't say that SIGBUS can occur without a page fault. The evaluation of whether a page would be beyond EOF, page allocation, and block allocation happen only in response to a page fault... > If a new fault is attempted then we have resource constraints that could > have caused a SIGBUS independently of the truncate. So that case is not > really something special to be considered for truncation. Agreed. I was just reacting to Jason's question whether an application cannot prevent page freeing by being aggressive enough. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
