Re: tmpfs loses (sub-page chunks of) data?

On Fri, Nov 15, 2024 at 02:43:22PM +0200, Andriy Gapon wrote:
> 
> We have a number of servers based on FreeSBD 13.3 that initially write some
> data to files on tmpfs and then the files are dispatched elsewhere.  The
> writes are done by appending variable sized records to a file.  There are no
> seeks or overwrites.
> 
> I observe that occasionally (very rarely indeed given the amount of data
> produced) we get a corrupted file.
> 
> In all cases so far the corruption follows the same pattern: data range from
> the end of a record until the next page-aligned boundary is zeroed out.
> That is, good data always continues from an offset which is multiple of 4096
> and the zeroed area never crosses such offsets.
> 
> Because of the page boundary, I have a suspicion that either tmpfs or,
> perhaps, the broader VM subsystem might have a race where writing to a page
> does not mark it dirty.  Maybe this is related to paging out of a tmpfs page
> to the swap.
> 
> The problem is that I have never been able to observe this happening, the
> corruption gets detected after the fact, hours after it occurs.
> 
> If anyone could suggest any areas / changes / techniques to explore the
> problem, I would be much obliged.

Do you have swap enabled on the problematic machines?

Are the files mapped, do you write or read through map?