On 2/19/24 18:17, Suren Baghdasaryan wrote: > On Thu, Feb 15, 2024 at 3:56 PM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: >> >> On Thu, Feb 15, 2024 at 06:27:29PM -0500, Steven Rostedt wrote: >> > All this, and we are still worried about 4k for useful debugging :-/ > > I was planning to refactor this function to print one record at a time > with a smaller buffer but after discussing with Kent, he has plans to > reuse this function and having the report in one buffer is needed for > that. We are printing to console, AFAICS all the code involved uses plain printk() I think it would be way easier to have a function using printk() for this use case than the seq_buf which is more suitable for /proc and friends. Then all concerns about buffers would be gone. It wouldn't be that much of a code duplication? >> Every additional 4k still needs justification. And whether we burn a >> reserve on this will have no observable effect on user output in >> remotely normal situations; if this allocation ever fails, we've already >> been in an OOM situation for awhile and we've already printed out this >> report many times, with less memory pressure where the allocation would >> have succeeded. > > I'm not sure this claim will always be true, specifically in the case > of low-end devices with relatively low amounts of reserves and in the That's right, GFP_ATOMIC failures can easily happen without prior OOMs. Consider a system where userspace allocations fill the memory as they usually do, up to high watermark. Then a burst of packets is received and handled by GFP_ATOMIC allocations that deplete the reserves and can't cause OOMs (OOM is when we fail to reclaim anything, but we are allocating from a context that can't reclaim), so the very first report would be an GFP_ATOMIC failure and now it can't allocate that buffer for printing. I'm sure more such scenarios exist, Cc: Tetsuo who I recall was an expert on this topic. > presence of a possible quick memory usage spike. We should also > consider a case when panic_on_oom is set. All we get is one OOM > report, so we get only one chance to capture this report. In any case, > I don't yet have data to prove or disprove this claim but it will be > interesting to test it with data from the field once the feature is > deployed. > > For now I think with Vlastimil's __GFP_NOWARN suggestion the code > becomes safe and the only risk is to lose this report. If we get cases > with reports missing this data, we can easily change to reserved > memory.
