Hi, I've just been looking at the following in engines/mmap.c: 69 #ifdef FIO_MADV_FREE 70 if (f->filetype == FIO_TYPE_BLOCK) 71 (void) posix_madvise(fmd->mmap_ptr, fmd->mmap_sz, FIO_MADV_FREE); 72 #endif I'm wondering about the rationale. On Linux FIO_MADV_FREE maps to MADV_REMOVE which worryingly has this to say (from http://man7.org/linux/man-pages/man2/madvise.2.html ): "MADV_REMOVE (since Linux 2.6.16) Free up a given range of pages and its associated backing store. This is equivalent to punching a hole in the corresponding byte range of the backing store (see fallocate(2)). [...]" So it's basically saying "this does a TRIM operation". https://github.com/axboe/fio/commit/3e10fb832645e3ab3ef006f589f0459dc567cb53 added the guard for only block devices because it was noticed that the posix_madvise call was punching holes but nonetheless do we still want to keep it for block devices. Things get even tricker on other platforms. On Solaris FIO_MADV_FREE maps to MADV_FREE and the man page from https://docs.oracle.com/cd/E36784_01/html/E36874/madvise-3c.html says this: "MADV_FREE Tell the kernel that contents in the specified address range are no longer important and the range will be overwritten. [...] This value cannot be used on mappings that have underlying file objects." So we really shouldn't be making this call on that platform. If you look at Linux's implementation of MADV_FREE it says this: MADV_FREE (since Linux 4.5) [...] The MADV_FREE operation can be applied only to private anonymous pages (see mmap(2)). [...] which seems to back up the warning it shouldn't be used with backing files. For the record this piece of code seems to have arrived with https://github.com/axboe/fio/commit/a1c58075279454a91ec43366846b93e8dcf9753c ("Add strong madvise() hint for cache pruning"). Would a patch just removing it be sensible? -- Sitsofe | http://sucs.org/~sits/ -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
