On (25/02/28 13:43), Ard Biesheuvel wrote: > On Fri, 28 Feb 2025 at 06:24, Sergey Senozhatsky > <senozhatsky@xxxxxxxxxxxx> wrote: > > > > On (25/02/26 14:00), David Sterba wrote: > > > What strikes me as alarming that you insert about 20 branches into a > > > realtime compression algorithm, where everything is basically a hot > > > path. Branches that almost never happen, and never if the output buffer > > > is big enough. > > > > > > Please drop the patch. > > > > David, just for educational purposes, there's only safe variant of lzo > > decompression, which seems to be doing a lot of NEED_OP (HAVE_OP) adding > > branches and so on, basically what Herbert is adding to the compression > > path. So my question is - why NEED_OP (if (!HAVE_OP(x)) goto output_overrun) > > is a no go for compression, but appears to be fine for decompression? > > > > Because compression has a bounded worst case (compressing data with > LZO can actually increase the size but only by a limited amount), > whereas decompressing a small input could produce gigabytes of output. One can argue that sometimes we know the upper bound. E.g. zswap and zram always compress physical pages, so decompression cannot result in a bigger (than the original physical page) data.
