Re: [PATCH] zsmalloc: fix a race with deferred_handles storing

Sergey Senozhatsky
2023-02-01T01:42:02Z

On (23/01/10 15:17), Nhat Pham wrote:
[..]
> #ifdef CONFIG_ZPOOL
> +static void restore_freelist(struct zs_pool *pool, struct size_class *class,
> + struct zspage *zspage)
> +{
> + unsigned int obj_idx = 0;
> + unsigned long handle, off = 0; /* off is within-page offset */
> + struct page *page = get_first_page(zspage);
> + struct link_free *prev_free = NULL;
> + void *prev_page_vaddr = NULL;
> +
> + /* in case no free object found */
> + set_freeobj(zspage, (unsigned int)(-1UL));

I'm not following this. I see how -1UL works for link_free, but this
cast of -1UL to 4 bytes looks suspicious.

> + while (page) {
> + void *vaddr = kmap_atomic(page);
> + struct page *next_page;
> +
> + while (off < PAGE_SIZE) {
> + void *obj_addr = vaddr + off;
> +
> + /* skip allocated object */
> + if (obj_allocated(page, obj_addr, &handle)) {
> + obj_idx++;
> + off += class->size;
> + continue;
> + }
> +
> + /* free deferred handle from reclaim attempt */
> + if (obj_stores_deferred_handle(page, obj_addr, &handle))
> + cache_free_handle(pool, handle);
> +
> + if (prev_free)
> + prev_free->next = obj_idx << OBJ_TAG_BITS;
> + else /* first free object found */
> + set_freeobj(zspage, obj_idx);
> +
> + prev_free = (struct link_free *)vaddr + off / sizeof(*prev_free);
> + /* if last free object in a previous page, need to unmap */
> + if (prev_page_vaddr) {
> + kunmap_atomic(prev_page_vaddr);
> + prev_page_vaddr = NULL;
> + }
> +
> + obj_idx++;
> + off += class->size;
> + }
> +
> + /*
> + * Handle the last (full or partial) object on this page.
> + */
> + next_page = get_next_page(page);
> + if (next_page) {
> + if (!prev_free || prev_page_vaddr) {
> + /*
> + * There is no free object in this page, so we can safely
> + * unmap it.
> + */
> + kunmap_atomic(vaddr);
> + } else {
> + /* update prev_page_vaddr since prev_free is on this page */
> + prev_page_vaddr = vaddr;
> + }

A polite and gentle nit: I'd appreciate it if we honored kernel coding
styles in zsmalloc a little bit more. Comments, function declarations, etc.
I'm personally very happy with https://github.com/vivien/vim-linux-coding-style