[..] > > > I guess that is because on arm32 , we have highmem but > > > sg_init_one supports lowmem only. the below should be > > > able to fix? > > > > > > diff --git a/mm/zswap.c b/mm/zswap.c > > > index 9dec853647c8..47c0386caba2 100644 > > > --- a/mm/zswap.c > > > +++ b/mm/zswap.c > > > @@ -1086,7 +1086,8 @@ static void zswap_decompress(struct zswap_entry > > > *entry, struct page *page) > > > zpool_unmap_handle(zpool, entry->handle); > > > } > > > > > > - sg_init_one(&input, src, entry->length); > > > + sg_init_table(&input, 1); > > > + sg_set_page(&input, kmap_to_page(src), entry->length, > > > offset_in_page(src)); > > > > Is this working around the debug check in sg_init_one()? IIUC, only > > I wouldn't characterize it as a workaround; it's more of a solution. I assumed that the debug check in sg_set_buf() is because sg_set_page() cannot handle highmem pages, sorry if that isn't the case. Apparently we are hitting a warning with kmap_to_page() though as syzbot just reported. > > > lowmem pages are supported. We may be passing in a highmem page to > > sg_set_page() now, right? > > we can pass highmem to sg_set_page(). This is perfectly fine. So the debug check is only because we are using virt_to_page() in sg_set_buf()? > > > > > Also, it seems like if src is a lowmem address kmap_to_page() will be > > doing unnecessary checks (assuming it's working correctly)? > > In practice, we consistently use kmap and kunmap even on systems with > low memory. > However, it's worth noting that for low memory scenarios, kmap > essentially returns > page_to_virt(page_address). Thus, the overhead of kmap_to_page shouldn't be > significant on low memory systems, especially considering that it simplifies to > virt_to_page(). > > Another approach is to consistently employ page_to_virt() for low > memory situations > and reserve kmap for high memory scenarios. However, since we always > utilize kmap > regardless of whether the page is low or high memory, we don't need to concern > ourselves with this distinction I see. Thanks for elaborating. > > > > > Would it be more robust to just use the temporary buffer if src is a > > kmap address? > > I don't think so because we will need a memcpy then. I thought that was necessary because sg_set_page() cannot take in highmem pages, but you mentioned that this isn't the case.
